[jun0811] WEEK 02 solutions

3sum/jun0811.js

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+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+
+// 중복 제거 중점..
+var threeSum = function (nums) {
+  nums.sort((a, b) => a - b);
+  const result = [];
+
+  for (let i = 0; i < nums.length - 2; i++) {
+    if (i > 0 && nums[i] === nums[i - 1]) continue;
+
+    let left = i + 1;
+    let right = nums.length - 1;
+
+    while (left < right) {
+      const sum = nums[i] + nums[left] + nums[right];
+
+      if (sum === 0) {
+        result.push([nums[i], nums[left], nums[right]]);
+
+        // 중복 미리 제거
+        while (left < right && nums[left] === nums[left + 1]) left++;
+        while (left < right && nums[right] === nums[right - 1]) right--;
+
+        left++;
+        right--;
+      } else if (sum < 0) {
+        left++;
+      } else {
+        right--;
+      }
+    }
+  }
+
+  return result;
+};

climbing-stairs/jun0811.js

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+/**
+ * @param {number} n
+ * @return {number}
+ */
+
+// Time Complexity : O(N)
+var climbStairs = function (n) {
+  if (n == 1) return 1;
+  if (n == 2) return 2;
+
+  const dp = [1, 2];
+
+  for (let i = 2; i < n; i++) {
+    dp[i] = dp[i - 2] + dp[i - 1];
+  }
+  return dp[n - 1];
+};

product-of-array-except-self/jun0811.js

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+/**
+ * @param {number[]} nums
+ * @return {number[]}
+ */
+var productExceptSelf = function (nums) {
+  const res = new Array(nums.length).fill(1);
+
+  for (let i = 1; i < nums.length; i++) {
+    res[i] = res[i - 1] * nums[i - 1];
+  }
+
+  let right = 1;
+  for (let i = nums.length - 1; i >= 0; i--) {
+    res[i] *= right;
+    right *= nums[i];
+  }
+
+  return res;
+};

valid-anagram/jun0811.js

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+/**
+ * @param {string} s
+ * @param {string} t
+ * @return {boolean}
+ */
+
+// Time Complexity : O(N)
+var isAnagram = function (s, t) {
+  if (s.length != t.length) return false;
+
+  const hashMap = {};
+
+  for (const chr of s) {
+    if (chr in hashMap) {
+      hashMap[chr] += 1;
+    } else {
+      hashMap[chr] = 1;
+    }
+  }
+
+  for (const chr of t) {
+    if (chr in hashMap && hashMap[chr] > 0) {
+      hashMap[chr] -= 1;
+    } else {
+      return false;
+    }
+  }
+
+  return true;
+};

validate-binary-search-tree/jun0811.js

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+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {boolean}
+ */
+function check(node, min, max) {
+  if (node === null) return true;
+  if ((min !== null && node.val <= min) || (max !== null && node.val >= max)) {
+    return false;
+  }
+
+  return check(node.left, min, node.val) && check(node.right, node.val, max);
+}
+
+function isValidBST(root) {
+  return check(root, null, null);
+}