[ganu] Week 2

3sum/gwbaik9717.js

@@ -0,0 +1,38 @@
+// Time complexity: O(n^2)
+// Space complexity: O(n)
+
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var threeSum = function (nums) {
+  const sumsDict = new Set();
+  const sortedNums = nums.toSorted((a, b) => a - b);
+
+  const n = nums.length;
+
+  for (let i = 0; i < n - 2; i++) {
+    let left = i + 1;
+    let right = n - 1;
+    const fixed = sortedNums[i];
+
+    const targetSum = 0 - fixed;
+
+    while (left < right) {
+      const currentSum = sortedNums[left] + sortedNums[right];
+
+      if (currentSum < targetSum) {
+        left++;
+      } else if (currentSum > targetSum) {
+        right--;
+      } else {
+        const key = [fixed, sortedNums[left], sortedNums[right]];
+        sumsDict.add(key.join(","));
+        left++;
+        right--;
+      }
+    }
+  }
+
+  return Array.from(sumsDict).map((nums) => nums.split(",").map(Number));
+};

climbing-stairs/gwbaik9717.js

@@ -0,0 +1,16 @@
+// Time complexity: O(n)
+// Space complexity: O(1)
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var climbStairs = function (n) {
+  const dp = [1, 1];
+
+  for (let i = 2; i <= n; i++) {
+    dp[i % 2] = dp[0] + dp[1];
+  }
+
+  return dp[n % 2];
+};

construct-binary-tree-from-preorder-and-inorder-traversal/gwbaik9717.js

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+// Time complexity: O(n^2)
+// Space complexity: O(n)
+
+/**
+ * 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 {number[]} preorder
+ * @param {number[]} inorder
+ * @return {TreeNode}
+ */
+var buildTree = function (preorder, inorder) {
+  if (!preorder.length || !inorder.length) return null;
+
+  const root = new TreeNode(preorder[0]);
+  const mid = inorder.indexOf(root.val);
+
+  root.left = buildTree(preorder.slice(1, mid + 1), inorder.slice(0, mid));
+  root.right = buildTree(preorder.slice(mid + 1), inorder.slice(mid + 1));
+
+  return root;
+};

decode-ways/gwbaik9717.js

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+// Time complexity: O(n)
+// Space complexity: O(n)
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var numDecodings = function (s) {
+  const n = s.length;
+  const dp = Array.from({ length: n + 2 }, () => 0);
+  dp[1] = 1;
+
+  for (let i = 2; i < n + 2; i++) {
+    // 한자리
+    const charCode = Number(s[i - 2]);
+
+    if (charCode > 0) {
+      dp[i] += dp[i - 1];
+    }
+
+    // 두자리
+    if (i <= 2) {
+      continue;
+    }
+
+    if (Number(s[i - 3]) == 0) {
+      continue;
+    }
+
+    const strCode = Number(s.slice(i - 3, i - 1));
+
+    if (strCode > 0 && strCode <= 26) {
+      dp[i] += dp[i - 2];
+    }
+  }
+
+  return dp.at(-1);
+};

valid-anagram/gwbaik9717.js

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+// Time complexity: O(n)
+// Space complexity: O(n)
+
+/**
+ * @param {string} s
+ * @param {string} t
+ * @return {boolean}
+ */
+var isAnagram = function (s, t) {
+  if (s.length !== t.length) {
+    return false;
+  }
+
+  const createDictFromString = (str) => {
+    const dict = new Map();
+
+    for (const chr of str) {
+      if (dict.has(chr)) {
+        dict.set(chr, dict.get(chr) + 1);
+        continue;
+      }
+
+      dict.set(chr, 1);
+    }
+
+    return dict;
+  };
+
+  const dictS = createDictFromString(s);
+  const dictT = createDictFromString(t);
+
+  for (const [key, value] of dictS) {
+    if (dictT.get(key) !== value) {
+      return false;
+    }
+  }
+
+  return true;
+};