permutations

Author: piyushkhemka

20. Valid Parentheses.cpp

@@ -0,0 +1,36 @@
+class Solution {
+public:
+    bool isValid(string s) {
+        stack<char> q;
+
+        for(int i = 0;i<s.length(); ++i) {
+            if(s[i] == '(' || s[i] == '{' || s[i] == '[')
+                q.push(s[i]);
+
+          else if(s[i] == ')') {
+                if(q.empty() || q.top() != '(')
+                    return false;
+
+                    q.pop();
+            }
+           else if(s[i] == '}') {
+                if(q.empty() || q.top() != '{')
+                    return false;
+
+                    q.pop();
+            }
+          else  if(s[i] == ']') {
+                if(q.empty() || q.top() != '[')
+                    return false;
+
+                    q.pop();
+            }
+        }
+
+        if(!q.empty())
+            return false;
+
+        return true;
+
+    }
+};

31. Next Permutation.cpp

@@ -0,0 +1,30 @@
+class Solution {
+public:
+    void nextPermutation(vector<int>& nums) {
+
+        if(nums.empty())
+            return;
+
+        if(nums.size() == 2) {
+            swap(nums[0],nums[1]);
+            return;
+        }
+
+        int i;
+        for(i = nums.size()-2;i>=0;i--) {
+            if(nums[i] < nums[i+1]) {
+                break;
+            }
+        }
+        reverse(nums.begin() + i+ 1, nums.end());
+        if(i == -1)
+            return;
+        auto nexthigh = upper_bound(nums.begin() + i + 1, nums.end(), nums[i]);
+     //   cout << *nexthigh;
+        swap(nums[i], *nexthigh);
+
+    //    sort(++nexthigh, nums.end());
+
+
+    }
+};

46. Permutations.cpp

@@ -0,0 +1,25 @@
+
+void perm(vector<int> &nums, vector<int> &row,vector<vector<int> > &result, int k) {
+    if (k == nums.size()-1) {
+        row = nums;
+        result.push_back(row);
+        return ;
+    }
+
+    for(int i = k; i<nums.size();++i) {
+        swap(nums[i],nums[k]);
+        perm(nums,row,result,k+1);
+        swap(nums[i],nums[k]);
+    }
+}
+
+class Solution {
+public:
+    vector<vector<int>> permute(vector<int>& nums) {
+
+        vector<int> row;
+        vector<vector<int> > result;
+        perm(nums,row,result,0);
+        return result;
+    }
+};