[gitsunmin] WEEK 2 Solution

construct-binary-tree-from-preorder-and-inorder-traversal/gitsunmin.ts

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+
+/**
+ * * 문제에서 정의된 타입
+ */
+export class TreeNode {
+    val: number
+    left: TreeNode | null
+    right: TreeNode | null
+    constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+        this.val = (val === undefined ? 0 : val)
+        this.left = (left === undefined ? null : left)
+        this.right = (right === undefined ? null : right)
+    }
+}
+
+/**
+ * ! 문제에서의 Output과 실제 정의된, 사용되는 output이 다르기 때문에, 한 번 변환 작업을 거처야함. (실제 제출 시 제외한 함수 입니다.)
+ */
+// function treeToArray(root: TreeNode | null): (number | null)[] {
+//     if (!root) return [];
+//     const result: (number | null)[] = [];
+//     const queue: (TreeNode | null)[] = [root];
+//     while (queue.length > 0) {
+//         const node = queue.shift();
+//         if (node) {
+//             result.push(node.val);
+//             queue.push(node.left);
+//             queue.push(node.right);
+//         } else {
+//             result.push(null);
+//         }
+//     }
+//     while (result[result.length - 1] === null) result.pop();
+//     return result;
+// }
+
+function buildTree(preorder: number[], inorder: number[]): TreeNode | null {
+    if (preorder.length === 0 || inorder.length === 0) return null;
+
+    const rootVal = preorder[0];
+    const inorderIndex = inorder.indexOf(rootVal);
+    const leftInorder = inorder.slice(0, inorderIndex);
+
+    return new TreeNode(
+        rootVal,
+        buildTree(
+            preorder.slice(1, 1 + leftInorder.length),
+            leftInorder
+        ),
+        buildTree(
+            preorder.slice(1 + leftInorder.length),
+            inorder.slice(inorderIndex + 1)
+        ),
+    );
+}
+
+
+// const preorder = [3, 9, 20, 15, 7];
+// const inorder = [9, 3, 15, 20, 7];
+// console.log('output:', treeToArray(buildTree(preorder, inorder))); 

counting-bits/gitsunmin.ts

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+/**
+ * https://leetcode.com/problems/counting-bits/
+ * time complexity : O(n)
+ * space complexity : O(n)
+ */
+function countBits(n: number): number[] {
+    const arr = new Array(n + 1).fill(0);
+    for (let i = 1; i <= n; i++) arr[i] = arr[i >> 1] + (i & 1);
+    return arr;
+}

encode-and-decode-strings/gitsunmin.ts

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+/**
+ * https://www.lintcode.com/problem/659/
+ * time complexity : O(n)
+ * space complexity : O(n)
+ * ! emoji는 입력에서 제외한다.
+ */
+
+function encode(input: Array<string>): string {
+    return input.join("😀");
+}
+
+function decode(input: string): Array<string> {
+    return input.split("😀");
+}

valid-anagram/gitsunmin.ts

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+/**
+ * https://leetcode.com/problems/valid-anagram/submissions
+ * time complexity : O(n)
+ * space complexity : O(n)
+ */
+function isAnagram(s: string, t: string): boolean {
+    if (s.length !== t.length) return false;
+
+    const map = {};
+
+    for (const char of s) map[char] = (map[char] ?? 0) + 1;
+
+    for (const char of t) {
+        if (map[char] !== undefined) {
+            map[char] = map[char] - 1;
+        } else return false;
+    }
+
+    for (const val of Object.values(map)) if (val !== 0) return false;
+
+    return true;
+};