diff --git a/include/common.h b/include/common.h
index 64fc69c..0ecc8a1 100644
--- a/include/common.h
+++ b/include/common.h
@@ -22,7 +22,7 @@ ListNode* build(const std::vector<int>& vec) {
     if (vec.empty()) return nullptr;
     ListNode* head = new ListNode(vec[0]);
     ListNode* cur = head;
-    for (int i = 1; i < vec.size(); i++) {
+    for (size_t i = 1; i < vec.size(); i++) {
         cur->next = new ListNode(vec[i]);
         cur = cur->next;
     }
diff --git a/src/1-twoSum.cpp b/src/1-twoSum.cpp
index db9f489..00a62e4 100644
--- a/src/1-twoSum.cpp
+++ b/src/1-twoSum.cpp
@@ -10,7 +10,7 @@ class Solution {
 public:
     vector<int> twoSum(vector<int>& nums, int target) {
         unordered_map<int, int> hash;
-        for (int i = 0; i < nums.size(); i++) {
+        for (int i = 0; i < static_cast<int>(nums.size()); i++) {
             if (hash.contains(target - nums[i])) {
                 return {hash[target - nums[i]], i};
             }
diff --git a/src/13-romanToInt.cpp b/src/13-romanToInt.cpp
index 879ee23..63126f5 100644
--- a/src/13-romanToInt.cpp
+++ b/src/13-romanToInt.cpp
@@ -20,8 +20,9 @@ class Solution {
         m['M'] = 1000;
 
         int res = 0;
-        for (int i = 0; i < s.size(); i++) {
-            if (i < s.size() - 1 && m[s[i]] < m[s[i + 1]]) {
+        const int n = static_cast<int>(s.size());
+        for (int i = 0; i < n; i++) {
+            if (i < n - 1 && m[s[i]] < m[s[i + 1]]) {
                 res -= m[s[i]];
             } else {
                 res += m[s[i]];
diff --git a/src/14-longestCommonPrefix.cpp b/src/14-longestCommonPrefix.cpp
index 1b7f940..cba5ab5 100644
--- a/src/14-longestCommonPrefix.cpp
+++ b/src/14-longestCommonPrefix.cpp
@@ -9,9 +9,10 @@ using namespace std;
 class Solution {
 public:
     string longestCommonPrefix(vector<string>& strs) {
-        for (int i = 0; i < strs[0].size(); i++) {
+        const int n = static_cast<int>(strs[0].size());
+        for (int i = 0; i < n; i++) {
             for (const auto& str : strs) {
-                if (i >= str.size() || str[i] != strs[0][i]) {
+                if (i >= n || str[i] != strs[0][i]) {
                     return strs[0].substr(0, i);
                 }
             }
diff --git a/src/15-threeSum.cpp b/src/15-threeSum.cpp
index ff01203..045e6c3 100644
--- a/src/15-threeSum.cpp
+++ b/src/15-threeSum.cpp
@@ -14,11 +14,12 @@ class Solution {
         }
         vector<vector<int>> res;
         ranges::sort(nums);
-        for (int i = 0; i < nums.size() - 2; i++) {
+        const int n = static_cast<int>(nums.size());
+        for (int i = 0; i < n - 2; i++) {
             if (i > 0 && nums[i] == nums[i - 1]) {
                 continue;
             }
-            int left = i + 1, right = nums.size() - 1;
+            int left = i + 1, right = n - 1;
             while (left < right) {
                 int sum = nums[i] + nums[left] + nums[right];
                 if (sum == 0) {
diff --git a/src/16-threeSumClosest.cpp b/src/16-threeSumClosest.cpp
index 6202323..f108dee 100644
--- a/src/16-threeSumClosest.cpp
+++ b/src/16-threeSumClosest.cpp
@@ -11,11 +11,12 @@ class Solution {
     int threeSumClosest(vector<int>& nums, int target) {
         int res = nums[0] + nums[1] + nums[2];
         ranges::sort(nums);
-        for (int i = 0; i < nums.size() - 2; i++) {
+        const int n = static_cast<int>(nums.size());
+        for (int i = 0; i < n - 2; i++) {
             if (i > 0 && nums[i] == nums[i - 1]) {
                 continue;
             }
-            int left = i + 1, right = nums.size() - 1;
+            int left = i + 1, right = n - 1;
             while (left < right) {
                 int sum = nums[i] + nums[left] + nums[right];
                 if (sum == target) {
diff --git a/src/17-letterCombinations.cpp b/src/17-letterCombinations.cpp
index 47bf918..a02dacc 100644
--- a/src/17-letterCombinations.cpp
+++ b/src/17-letterCombinations.cpp
@@ -21,7 +21,7 @@ class Solution {
     const vector<string> m{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
 
     void dfs(const string& digits, int index, string&& path, vector<string>& res) {
-        if (index == digits.size()) {
+        if (index == static_cast<int>(digits.size())) {
             res.push_back(path);
             return;
         }
diff --git a/src/18-fourSum.cpp b/src/18-fourSum.cpp
index 21d946c..ac00f0d 100644
--- a/src/18-fourSum.cpp
+++ b/src/18-fourSum.cpp
@@ -16,10 +16,11 @@ class Solution {
 
 private:
     vector<vector<int>> kSum(vector<int>& nums, int k, int start, long target) {
-        if (nums.size() < k || k < 2) return {};
+        const int n = static_cast<int>(nums.size());
+        if (n < k || k < 2) return {};
         vector<vector<int>> res;
         if (k == 2) {
-            int left = start, right = nums.size() - 1;
+            int left = start, right = n - 1;
             while (left < right) {
                 long sum = nums[left] + nums[right];
                 if (sum == target) {
@@ -33,13 +34,13 @@ class Solution {
                 }
             }
         } else {
-            for (int i = start; i < nums.size(); i++) {
+            for (int i = start; i < n; i++) {
                 auto sub = kSum(nums, k - 1, i + 1, target - nums[i]);
                 for (auto& v : sub) {
                     v.push_back(nums[i]);
                     res.push_back(v);
                 }
-                while (i < nums.size() - 1 && nums[i] == nums[i + 1]) i++;
+                while (i < n - 1 && nums[i] == nums[i + 1]) i++;
             }
         }
 
diff --git a/src/26-removeDuplicates.cpp b/src/26-removeDuplicates.cpp
index 96980d8..485838f 100644
--- a/src/26-removeDuplicates.cpp
+++ b/src/26-removeDuplicates.cpp
@@ -11,7 +11,7 @@ class Solution {
     int removeDuplicates(vector<int>& nums) {
         if (nums.empty()) return 0;
         int res = 0;
-        for (int i = 1; i < nums.size(); i++) {
+        for (int i = 1; i < static_cast<int>(nums.size()); i++) {
             if (nums[res] != nums[i]) {
                 nums[++res] = nums[i];
             }
@@ -24,12 +24,12 @@ void test() {
     Solution s;
     vector<int> nums = {1, 1, 2};
     vector<int> expected = {1, 2};
-    assert(s.removeDuplicates(nums) == expected.size());
+    assert(s.removeDuplicates(nums) == static_cast<int>(expected.size()));
     assert(vector<int>(nums.begin(), nums.begin() + expected.size()) == expected);
 
     nums = {0, 0, 1, 1, 1, 2, 2, 3, 3, 4};
     expected = {0, 1, 2, 3, 4};
-    assert(s.removeDuplicates(nums) == expected.size());
+    assert(s.removeDuplicates(nums) == static_cast<int>(expected.size()));
     assert(vector<int>(nums.begin(), nums.begin() + expected.size()) == expected);
 }
 
diff --git a/src/27-removeElement.cpp b/src/27-removeElement.cpp
index 027ed94..005c769 100644
--- a/src/27-removeElement.cpp
+++ b/src/27-removeElement.cpp
@@ -11,7 +11,7 @@ class Solution {
     int removeElement(vector<int>& nums, int val) {
         if (nums.empty()) return 0;
         int res = -1;
-        for (int i = 0; i < nums.size(); i++) {
+        for (int i = 0; i < static_cast<int>(nums.size()); i++) {
             if (nums[i] != val) {
                 nums[++res] = nums[i];
             }
@@ -24,17 +24,17 @@ void test() {
     Solution s;
     vector<int> nums = {3, 2, 2, 3};
     vector<int> expected = {2, 2};
-    assert(s.removeElement(nums, 3) == expected.size());
+    assert(s.removeElement(nums, 3) == static_cast<int>(expected.size()));
     assert(vector<int>(nums.begin(), nums.begin() + expected.size()) == expected);
 
     nums = {0, 1, 2, 2, 3, 0, 4, 2};
     expected = {0, 1, 3, 0, 4};
-    assert(s.removeElement(nums, 2) == expected.size());
+    assert(s.removeElement(nums, 2) == static_cast<int>(expected.size()));
     assert(vector<int>(nums.begin(), nums.begin() + expected.size()) == expected);
 
     nums = {};
     expected = {};
-    assert(s.removeElement(nums, 1) == expected.size());
+    assert(s.removeElement(nums, 1) == static_cast<int>(expected.size()));
     assert(vector<int>(nums.begin(), nums.begin() + expected.size()) == expected);
 }
 
diff --git a/src/3-lengthOfLongestSubstring.cpp b/src/3-lengthOfLongestSubstring.cpp
index b866911..7406335 100644
--- a/src/3-lengthOfLongestSubstring.cpp
+++ b/src/3-lengthOfLongestSubstring.cpp
@@ -9,12 +9,15 @@ using namespace std;
 class Solution {
 public:
     int lengthOfLongestSubstring(string s) {
-        char chs[128] = {0};
+        int chs[128] = {0};
         int res = 0;
-        for (int left = 0, right = 0; right < s.size(); right++) {
-            chs[s[right]]++;
-            while (chs[s[right]] > 1) {
-                chs[s[left++]]--;
+        for (int left = 0, right = 0; right < static_cast<int>(s.size()); right++) {
+            auto c = static_cast<unsigned char>(s[right]);
+            chs[c]++;
+            while (chs[c] > 1) {
+                auto lc = static_cast<unsigned char>(s[left]);
+                chs[lc]--;
+                left++;
             }
             res = max(res, right - left + 1);
         }
diff --git a/src/34-searchRange.cpp b/src/34-searchRange.cpp
index 446d379..7416dd4 100644
--- a/src/34-searchRange.cpp
+++ b/src/34-searchRange.cpp
@@ -10,7 +10,7 @@ class Solution {
 public:
     vector<int> searchRange(vector<int>& nums, int target) {
         const int l = static_cast<int>(ranges::lower_bound(nums, target) - nums.begin());  // find first >= target
-        if (l == nums.size() || nums[l] != target) return {-1, -1};
+        if (l == static_cast<int>(nums.size()) || nums[l] != target) return {-1, -1};
         const int r = static_cast<int>(ranges::upper_bound(nums, target) - nums.begin() - 1);  // find first > target
         return {l, r};
     }
diff --git a/src/38-countAndSay.cpp b/src/38-countAndSay.cpp
index 9ac130c..a6e9036 100644
--- a/src/38-countAndSay.cpp
+++ b/src/38-countAndSay.cpp
@@ -12,9 +12,10 @@ class Solution {
         string res = "1";
         while (--n) {
             string tmp;
-            for (int i = 0; i < res.size(); ++i) {
+            const int m = static_cast<int>(res.size());
+            for (int i = 0; i < m; ++i) {
                 int cnt = 1;
-                while (i < res.size() - 1 && res[i] == res[i + 1]) {
+                while (i < m - 1 && res[i] == res[i + 1]) {
                     cnt++;
                     i++;
                 }
diff --git a/src/39-combinationSum.cpp b/src/39-combinationSum.cpp
index 6fb92d5..f215a61 100644
--- a/src/39-combinationSum.cpp
+++ b/src/39-combinationSum.cpp
@@ -21,7 +21,7 @@ class Solution {
             res.emplace_back(path);
             return;
         }
-        for (int i = index; i < candidates.size(); i++) {
+        for (int i = index; i < static_cast<int>(candidates.size()); i++) {
             if (candidates[i] > target) break;
             path.emplace_back(candidates[i]);
             dfs(candidates, target - candidates[i], i, std::move(path), res);
diff --git a/src/40-combinationSum2.cpp b/src/40-combinationSum2.cpp
index 71f549e..f4ee6bf 100644
--- a/src/40-combinationSum2.cpp
+++ b/src/40-combinationSum2.cpp
@@ -21,7 +21,7 @@ class Solution {
             res.emplace_back(path);
             return;
         }
-        for (int i = index; i < candidates.size(); i++) {
+        for (int i = index; i < static_cast<int>(candidates.size()); i++) {
             if (candidates[i] > target) break;
             if (i > index && candidates[i] == candidates[i - 1]) continue;  // 剪枝 重点理解这个 i>index 的作用
             path.emplace_back(candidates[i]);
diff --git a/src/43-multiply.cpp b/src/43-multiply.cpp
index 9bbf24a..176686b 100644
--- a/src/43-multiply.cpp
+++ b/src/43-multiply.cpp
@@ -10,15 +10,16 @@ class Solution {
 public:
     string multiply(string num1, string num2) {
         string res(num1.size() + num2.size(), '0');
-        for (int i = num1.size() - 1; i >= 0; i--) {
-            for (int j = num2.size() - 1; j >= 0; j--) {
+        const int m = static_cast<int>(num1.size()), n = static_cast<int>(num2.size());
+        for (int i = m - 1; i >= 0; i--) {
+            for (int j = n - 1; j >= 0; j--) {
                 const int mult = (num1[i] - '0') * (num2[j] - '0');
                 const int sum = mult + (res[i + j + 1] - '0');
-                res[i + j + 1] = (sum % 10) + '0';
+                res[i + j + 1] = static_cast<char>((sum % 10) + '0');
                 res[i + j] += sum / 10;
             }
         }
-        const int idx = res.find_first_not_of('0');
+        auto idx = res.find_first_not_of('0');
         return idx == string::npos ? "0" : res.substr(idx);
     }
 };
diff --git a/src/45-jump.cpp b/src/45-jump.cpp
index bdd58fa..fedaee2 100644
--- a/src/45-jump.cpp
+++ b/src/45-jump.cpp
@@ -12,9 +12,10 @@ class Solution {
         int res = 0;
         int end = 0;
         int farthest = 0;
-        for (int i = 0; i < nums.size() - 1; i++) {
+        const int n = static_cast<int>(nums.size());
+        for (int i = 0; i < n - 1; i++) {
             farthest = max(farthest, i + nums[i]);
-            if (farthest >= nums.size() - 1) {
+            if (farthest >= n - 1) {
                 res++;
                 break;
             }
diff --git a/src/46-permute.cpp b/src/46-permute.cpp
index 96b92d0..9a67a07 100644
--- a/src/46-permute.cpp
+++ b/src/46-permute.cpp
@@ -20,7 +20,7 @@ class Solution {
             res.emplace_back(path);
             return;
         }
-        for (int i = 0; i < nums.size(); ++i) {
+        for (int i = 0; i < static_cast<int>(nums.size()); ++i) {
             if (used[i]) continue;
             used[i] = true;
             path.emplace_back(nums[i]);
diff --git a/src/47-permuteUnique.cpp b/src/47-permuteUnique.cpp
index 5982fcf..93c3b8e 100644
--- a/src/47-permuteUnique.cpp
+++ b/src/47-permuteUnique.cpp
@@ -21,7 +21,7 @@ class Solution {
             res.emplace_back(path);
             return;
         }
-        for (int i = 0; i < nums.size(); ++i) {
+        for (int i = 0; i < static_cast<int>(nums.size()); ++i) {
             if (used[i]) continue;
             if (i > 0 && nums[i] == nums[i - 1] && !used[i - 1]) continue;
             used[i] = true;
diff --git a/src/48-rotate.cpp b/src/48-rotate.cpp
index 7ae4303..fbb3579 100644
--- a/src/48-rotate.cpp
+++ b/src/48-rotate.cpp
@@ -9,8 +9,8 @@ using namespace std;
 class Solution {
 public:
     void rotate(vector<vector<int>>& matrix) {
-        int m = matrix.size();
-        int n = matrix[0].size();
+        const int m = matrix.size();
+        const int n = matrix[0].size();
         for (int i = 0; i < m; i++) {
             for (int j = i; j < n; j++) {
                 swap(matrix[i][j], matrix[j][i]);  // 转置
diff --git a/src/5-longestPalindrome.cpp b/src/5-longestPalindrome.cpp
index 914c699..41b8169 100644
--- a/src/5-longestPalindrome.cpp
+++ b/src/5-longestPalindrome.cpp
@@ -10,7 +10,7 @@ class Solution {
 public:
     string longestPalindrome(string s) {
         pair<int, int> index{0, 0};
-        for (int i = 0; i < s.size(); i++) {
+        for (int i = 0; i < static_cast<int>(s.size()); i++) {
             auto index1 = findPalindrome(s, i, i);
             auto index2 = findPalindrome(s, i, i + 1);
             index = index1.second - index1.first > index.second - index.first ? index1 : index;
@@ -21,7 +21,7 @@ class Solution {
 
 private:
     static pair<int, int> findPalindrome(string s, int left, int right) {
-        while (left >= 0 && right < s.size() && s[left] == s[right]) {
+        while (left >= 0 && right < static_cast<int>(s.size()) && s[left] == s[right]) {
             left--;
             right++;
         }
diff --git a/src/53-maxSubArray.cpp b/src/53-maxSubArray.cpp
index 4555711..d78b704 100644
--- a/src/53-maxSubArray.cpp
+++ b/src/53-maxSubArray.cpp
@@ -11,7 +11,7 @@ class Solution {
     int maxSubArray(vector<int>& nums) {
         int res = INT_MIN;
         int sum = 0;
-        for (int i = 0; i < nums.size(); i++) {
+        for (int i = 0; i < static_cast<int>(nums.size()); i++) {
             sum += nums[i];
             res = max(res, sum);
             if (sum < 0) {
diff --git a/src/54-spiralOrder.cpp b/src/54-spiralOrder.cpp
index 81ebffc..babf459 100644
--- a/src/54-spiralOrder.cpp
+++ b/src/54-spiralOrder.cpp
@@ -9,12 +9,12 @@ using namespace std;
 class Solution {
 public:
     vector<int> spiralOrder(vector<vector<int>>& matrix) {
-        int m = matrix.size();
-        int n = matrix[0].size();
+        const int m = static_cast<int>(matrix.size());
+        const int n = static_cast<int>(matrix[0].size());
         vector<int> res;
         res.reserve(m * n);
         int i = 0, j = 0;
-        while (res.size() < m * n) {
+        while (static_cast<int>(res.size()) < m * n) {
             while (i >= 0 && i < m && j >= 0 && j < n && matrix[i][j] != INT_MIN) {
                 res.push_back(matrix[i][j]);
                 matrix[i][j] = INT_MIN;
diff --git a/src/55-canJump.cpp b/src/55-canJump.cpp
index c2f0108..efac5f6 100644
--- a/src/55-canJump.cpp
+++ b/src/55-canJump.cpp
@@ -10,9 +10,10 @@ class Solution {
 public:
     bool canJump(vector<int>& nums) {
         int reach = 0;
-        for (int i = 0; i < nums.size() && i <= reach; ++i) {
+        const int n = static_cast<int>(nums.size());
+        for (int i = 0; i < n && i <= reach; ++i) {
             reach = max(reach, i + nums[i]);
-            if (reach >= nums.size() - 1) return true;
+            if (reach >= n - 1) return true;
         }
         return false;
     }
diff --git a/src/6-convert.cpp b/src/6-convert.cpp
index 158867e..a0d538a 100644
--- a/src/6-convert.cpp
+++ b/src/6-convert.cpp
@@ -13,7 +13,7 @@ class Solution {
             return s;
         }
         vector<vector<char>> m(numRows);
-        for (int i = 0, row = 0, dir = 1; i < s.size(); i++) {
+        for (int i = 0, row = 0, dir = 1; i < static_cast<int>(s.size()); i++) {
             if (dir == 1) {
                 m[row++].push_back(s[i]);
             } else {
diff --git a/src/64-minPathSum.cpp b/src/64-minPathSum.cpp
index b08e365..2dfbf21 100644
--- a/src/64-minPathSum.cpp
+++ b/src/64-minPathSum.cpp
@@ -9,8 +9,8 @@ using namespace std;
 class Solution {
 public:
     int minPathSum(vector<vector<int>>& grid) {
-        const int m = grid.size();
-        const int n = grid[0].size();
+        const int m = static_cast<int>(grid.size());
+        const int n = static_cast<int>(grid[0].size());
         vector<vector<int>> dp(m, vector<int>(n, 0));
         dp[0][0] = grid[0][0];
         for (int i = 1; i < m; ++i) {
diff --git a/src/66-plusOne.cpp b/src/66-plusOne.cpp
index 9579942..d50b7d5 100644
--- a/src/66-plusOne.cpp
+++ b/src/66-plusOne.cpp
@@ -9,7 +9,7 @@ using namespace std;
 class Solution {
 public:
     vector<int> plusOne(vector<int>& digits) {
-        for (int i = digits.size() - 1; i >= 0; --i) {
+        for (int i = static_cast<int>(digits.size()) - 1; i >= 0; --i) {
             if (digits[i] < 9) {
                 digits[i]++;
                 return digits;
diff --git a/src/67-addBinary.cpp b/src/67-addBinary.cpp
index eade124..9895573 100644
--- a/src/67-addBinary.cpp
+++ b/src/67-addBinary.cpp
@@ -10,12 +10,12 @@ class Solution {
 public:
     string addBinary(string a, string b) {
         string res;
-        int i = a.size() - 1, j = b.size() - 1, carry = 0;
+        int i = static_cast<int>(a.size()) - 1, j = static_cast<int>(b.size()) - 1, carry = 0;
         while (i >= 0 || j >= 0 || carry) {
             const int x = i >= 0 ? a[i--] - '0' : 0;
             const int y = j >= 0 ? b[j--] - '0' : 0;
             const int sum = x + y + carry;
-            res.push_back(sum % 2 + '0');
+            res.push_back(static_cast<char>(sum % 2 + '0'));
             carry = sum / 2;
         }
         ranges::reverse(res);
diff --git a/src/69-mySqrt.cpp b/src/69-mySqrt.cpp
index 55abd5c..a637c40 100644
--- a/src/69-mySqrt.cpp
+++ b/src/69-mySqrt.cpp
@@ -15,7 +15,7 @@ class Solution {
         int left = 1, right = x / 2;
         int res = 0;
         while (left <= right) {
-            int mid = left + ((right - left) >> 1);
+            const int mid = left + ((right - left) >> 1);
             if (mid <= x / mid) {
                 res = mid;
                 left = mid + 1;
diff --git a/src/74-searchMatrix.cpp b/src/74-searchMatrix.cpp
index b8468ca..01936b1 100644
--- a/src/74-searchMatrix.cpp
+++ b/src/74-searchMatrix.cpp
@@ -9,8 +9,8 @@ using namespace std;
 class Solution {
 public:
     bool searchMatrix(vector<vector<int>>& matrix, int target) {
-        const int m = matrix.size();
-        const int n = matrix[0].size();
+        const int m = static_cast<int>(matrix.size());
+        const int n = static_cast<int>(matrix[0].size());
         int i = 0, j = n - 1;
         while (i >= 0 && i < m && j >= 0 && j < n) {
             if (matrix[i][j] == target) {
diff --git a/src/77-combine.cpp b/src/77-combine.cpp
index 11669d2..5fb8095 100644
--- a/src/77-combine.cpp
+++ b/src/77-combine.cpp
@@ -17,7 +17,7 @@ class Solution {
 
 private:
     void dfs(vector<vector<int>>& res, vector<int>&& path, int start, int n, int k) {
-        if (path.size() == k) {
+        if (static_cast<int>(path.size()) == k) {
             res.emplace_back(path);
             return;
         }
diff --git a/src/78-subsets.cpp b/src/78-subsets.cpp
index 8f8a6fc..a9733a1 100644
--- a/src/78-subsets.cpp
+++ b/src/78-subsets.cpp
@@ -18,7 +18,7 @@ class Solution {
 private:
     void dfs(vector<int>& nums, int index, vector<int>&& path, vector<vector<int>>& res) {
         res.emplace_back(path);
-        for (int i = index; i < nums.size(); i++) {
+        for (int i = index; i < static_cast<int>(nums.size()); i++) {
             path.emplace_back(nums[i]);
             dfs(nums, i + 1, std::move(path), res);
             path.pop_back();
diff --git a/src/79-exist.cpp b/src/79-exist.cpp
index 672d609..922f9c4 100644
--- a/src/79-exist.cpp
+++ b/src/79-exist.cpp
@@ -26,8 +26,8 @@ class Solution {
         if (path == word) {
             return true;
         }
-        if (i < 0 || i >= board.size() || j < 0 || j >= board[0].size() || search[i][j] ||
-            board[i][j] != word[path.size()]) {
+        if (i < 0 || i >= static_cast<int>(board.size()) || j < 0 || j >= static_cast<int>(board[0].size()) ||
+            search[i][j] || board[i][j] != word[path.size()]) {
             return false;
         }
         search[i][j] = true;
diff --git a/src/8-myAtoi.cpp b/src/8-myAtoi.cpp
index c9947e1..0f664e0 100644
--- a/src/8-myAtoi.cpp
+++ b/src/8-myAtoi.cpp
@@ -9,7 +9,7 @@ using namespace std;
 class Solution {
 public:
     int myAtoi(string s) {
-        int i = s.find_first_not_of(' ');
+        size_t i = s.find_first_not_of(' ');
         if (i == string::npos) return 0;
         const int sign = s[i] == '-' ? -1 : 1;
         if (s[i] == '-' || s[i] == '+') i++;
diff --git a/src/80-removeDuplicates.cpp b/src/80-removeDuplicates.cpp
index 778b3ba..bee5f76 100644
--- a/src/80-removeDuplicates.cpp
+++ b/src/80-removeDuplicates.cpp
@@ -23,12 +23,12 @@ void test() {
     Solution s;
     vector<int> nums = {1, 1, 1, 2, 2, 3};
     vector<int> expected = {1, 1, 2, 2, 3};
-    assert(s.removeDuplicates(nums) == expected.size());
+    assert(s.removeDuplicates(nums) == static_cast<int>(expected.size()));
     assert(vector<int>(nums.begin(), nums.begin() + expected.size()) == expected);
 
     nums = {0, 0, 1, 1, 1, 1, 2, 3, 3};
     expected = {0, 0, 1, 1, 2, 3, 3};
-    assert(s.removeDuplicates(nums) == expected.size());
+    assert(s.removeDuplicates(nums) == static_cast<int>(expected.size()));
     assert(vector<int>(nums.begin(), nums.begin() + expected.size()) == expected);
 }
 
diff --git a/src/90-subsetsWithDup.cpp b/src/90-subsetsWithDup.cpp
index f144d69..37bea98 100644
--- a/src/90-subsetsWithDup.cpp
+++ b/src/90-subsetsWithDup.cpp
@@ -19,7 +19,7 @@ class Solution {
 private:
     void dfs(vector<int>& nums, int index, vector<int>&& path, vector<vector<int>>& res) {
         res.emplace_back(path);
-        for (int i = index; i < nums.size(); ++i) {
+        for (int i = index; i < static_cast<int>(nums.size()); ++i) {
             if (i > index && nums[i] == nums[i - 1]) {
                 continue;
             }
diff --git a/src/91-numDecodings.cpp b/src/91-numDecodings.cpp
index 2ac2ec3..a0e85bd 100644
--- a/src/91-numDecodings.cpp
+++ b/src/91-numDecodings.cpp
@@ -16,7 +16,7 @@ class Solution {
         } else {
             dp[1] = 1;
         }
-        for (int i = 2; i <= s.size(); i++) {
+        for (int i = 2; i <= static_cast<int>(s.size()); i++) {
             if (s[i - 1] != '0') {
                 dp[i] += dp[i - 1];
             }