[선재] WEEK02 Solutions

construct-binary-tree-from-preorder-and-inorder-traversal/sunjae95.js

@@ -0,0 +1,45 @@
+/**
+ * @description
+ * time complexity: O(n^2)
+ * space complexity: O(n)
+ *
+ * brainstorming:
+ * stack, Drawing a graph
+ *
+ * strategy:
+ * discover the rules
+ * leftStack = left create , rightStack = right create
+ */
+var buildTree = function (preorder, inorder) {
+  let answer = null;
+  let pointer = 0;
+
+  const leftStack = [];
+  const rightStack = [];
+
+  preorder.forEach((val, i) => {
+    const node = new TreeNode(val);
+
+    if (i === 0) answer = node;
+
+    const leftLen = leftStack.length;
+    const rightLen = rightStack.length;
+
+    if (leftLen && rightLen) {
+      if (leftStack[leftLen - 1].left) rightStack[rightLen - 1].right = node;
+      else leftStack[leftLen - 1].left = node;
+    }
+    if (leftLen && !rightLen) leftStack[leftLen - 1].left = node;
+    if (!leftLen && rightLen) rightStack[rightLen - 1].right = node;
+
+    leftStack.push(node);
+
+    while (leftStack.length && pointer < inorder.length) {
+      if (leftStack[leftStack.length - 1].val !== inorder[pointer]) break;
+      rightStack.push(leftStack.pop());
+      pointer++;
+    }
+  });
+
+  return answer;
+};

counting-bits/sunjae95.js

@@ -0,0 +1,24 @@
+/**
+ * @description
+ * time complexity: O(n log n)
+ * space complexity: O(N)
+ *
+ * brainstorming:
+ * convert integer to bit
+ * for loop
+ *
+ * strategy:
+ * string change to hash table
+ */
+var countBits = function (n) {
+  return Array.from({ length: n + 1 }, (_, i) => convertBitCount(i));
+};
+
+const convertBitCount = (n) => {
+  let count = 0;
+  while (n > 0) {
+    count += n % 2;
+    n = Math.floor(n / 2);
+  }
+  return count;
+};

decode-ways/sunjae95.js

@@ -0,0 +1,76 @@
+/**
+ * @description
+ * brainstorming:
+ * 1. dp -> dp[i] = dp[i-1] + count
+ * 2. recursive function
+ *
+ * strategy:
+ * https://www.algodale.com/problems/decode-ways/
+ *
+ * result:
+ * 1. couldn't think of the conditions
+ *      true: 1~9, 10~27
+ *      false: 0, 0N, 28↑
+ * 2. persist solution that is top down
+ */
+
+// https://www.algodale.com/problems/decode-ways/ Solve 1
+/**
+ * time complexity: O(2^n)
+ * space complexity: O(n)
+ */
+var numDecodings = function (s) {
+  const search = (start) => {
+    if (start === s.length) return 1;
+    if (s[start] === "0") return 0;
+    if (start + 1 < s.length && Number(`${s[start]}${s[start + 1]}`) < 27) {
+      return search(start + 1) + search(start + 2);
+    }
+    return search(start + 1);
+  };
+
+  return search(0);
+};
+
+// https://www.algodale.com/problems/decode-ways/ Solve 2
+/**
+ * time complexity: O(2^n)
+ * space complexity: O(n)
+ */
+var numDecodings = function (s) {
+  const memo = new Map();
+  memo.set(s.length, 1);
+
+  const search = (start) => {
+    if (!!memo.get(start)) return memo.get(start);
+
+    if (s[start] === "0") memo.set(start, 0);
+    else if (start + 1 < s.length && Number(`${s[start]}${s[start + 1]}`) < 27)
+      memo.set(start, search(start + 1) + search(start + 2));
+    else memo.set(start, search(start + 1));
+
+    return memo.get(start);
+  };
+
+  return search(0);
+};
+
+// https://www.algodale.com/problems/decode-ways/ Solve 3
+/**
+ * time complexity: O(n)
+ * space complexity: O(n)
+ */
+var numDecodings = function (s) {
+  const dp = Array.from({ length: s.length + 1 }, (_, i) =>
+    i === s.length ? 1 : 0
+  );
+
+  for (let i = s.length - 1; i >= 0; i--) {
+    if (s[i] === "0") dp[i] = 0;
+    else if (i + 1 < s.length && Number(`${s[i]}${s[i + 1]}`) < 27)
+      dp[i] = dp[i + 1] + dp[i + 2];
+    else dp[i] = dp[i + 1];
+  }
+
+  return dp[0];
+};

palindromic-substrings/sunjae95.js

@@ -88,3 +88,77 @@ var countSubstrings = function (s) {
 
   return answer.length;
 };
+
+/**
+ * @description
+ * WEEK 01 Feedback
+ * 1. remove slice and change to endpoint
+ * 2. kadane's algorithm
+ *
+ * time complexity: O(N^3)
+ * space complexity: O(1)
+ *
+ * result
+ * solution 1 apply
+ */
+var countSubstrings = function (s) {
+  let answer = 0;
+  const len = s.length;
+
+  for (let startIndex = 0; startIndex < len; startIndex++) {
+    let subStr = "";
+    let isSubPalindromic = true;
+    answer++;
+
+    for (let endIndex = startIndex + 1; endIndex < len; endIndex++) {
+      if (isSubPalindromic && s[startIndex] === s[endIndex]) answer++;
+      subStr += s[endIndex];
+      isSubPalindromic = isPalindromic(subStr);
+    }
+  }
+
+  return answer;
+};
+
+function isPalindromic(str) {
+  const len = str.length;
+  const middleIndex = Math.floor(len / 2);
+
+  for (let i = 0; i < middleIndex; i++) {
+    if (str[i] !== str[len - 1 - i]) return false;
+  }
+
+  return true;
+}
+/**
+ * @description
+ * WEEK 01 Feedback
+ * 1. remove slice and change to endpoint
+ * 2. kadane's algorithm
+ *
+ * time complexity: O(N^2)
+ * space complexity: O(1)
+ *
+ * result
+ * solve 3(https://www.algodale.com/problems/palindromic-substrings/) include 1,2 solution
+ */
+var countSubstrings = function (s) {
+  let count = 0;
+  const len = s.length;
+
+  for (let i = 0; i < len; i++) {
+    let [start, end] = [i, i];
+
+    while (s[start] === s[end] && start >= 0 && end < len) {
+      count++;
+      [start, end] = [start - 1, end + 1];
+    }
+    [start, end] = [i, i + 1];
+    while (s[start] === s[end] && start >= 0 && end < len) {
+      count++;
+      [start, end] = [start - 1, end + 1];
+    }
+  }
+
+  return count;
+};

valid-anagram/sunjae95.js

@@ -0,0 +1,31 @@
+/**
+ * @description
+ * time complexity: O(N)
+ * space complexity: O(N)
+ *
+ * brainstorming:
+ * 1. hash table value compare to count
+ *
+ * strategy:
+ * string change to hash table
+ */
+var isAnagram = function (s, t) {
+  if (s.length !== t.length) return false;
+
+  let answer = true;
+  const tableS = convertHashTable(s);
+  const tableT = convertHashTable(t);
+
+  tableS.forEach((_, key) => {
+    if (tableT.get(key) && tableT.get(key) === tableS.get(key)) return;
+    answer = false;
+  });
+
+  return answer;
+};
+
+const convertHashTable = (str) =>
+  str.split("").reduce((map, s) => {
+    map.set(s, (map.get(s) ?? 0) + 1);
+    return map;
+  }, new Map());