[gitsunmin] WEEK 01 Solutions

contains-duplicate/gitsunmin.ts

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+/**
+ * https://leetcode.com/problems/contains-duplicate/
+ * time complexity : O(n)
+ * space complexity : O(n)
+ */
+function containsDuplicate(nums: number[]): boolean {
+    return new Set(nums).size !== nums.length
+};

kth-smallest-element-in-a-bst/gitsunmin.ts

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+/**
+ * https://leetcode.com/problems/kth-smallest-element-in-a-bst/
+ * time complexity : O(n)
+ * space complexity : O(n)
+ */
+
+/**
+ * * 문제에서 정의된 타입
+ */
+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 Input과 실제 정의된, 사용되는 input이 다르기 때문에, 한 번 변환 작업을 거처야함. (실제 제출 시 제외한 함수 입니다.)
+ */
+const arrayToTreeNode = (arr: Array<number | null>) => (k: number): [TreeNode | null, number] => {
+    let root = new TreeNode(arr[0]!);
+    let queue: (TreeNode | null)[] = [root];
+    let i = 1;
+
+    while (i < arr.length) {
+        let current = queue.shift();
+        if (current !== null && current !== undefined) {
+            if (arr[i] !== null) {
+                current.left = new TreeNode(arr[i]!);
+                queue.push(current.left);
+            }
+            i++;
+            if (i < arr.length && arr[i] !== null) {
+                current.right = new TreeNode(arr[i]!);
+                queue.push(current.right);
+            }
+            i++;
+        }
+    }
+
+    return [root, k];
+}
+
+// const input1 = arrayToTreeNode([3, 1, 4, null, 2]);
+// const input2 = arrayToTreeNode([5, 3, 6, 2, 4, null, null, 1]);
+
+// const output1 = kthSmallest(...input1(1))
+// const output2 = kthSmallest(...input2(3))
+
+// console.log('output1:', output1);
+// console.log('output2:', output2);
+
+
+function inOrderTraversal(node: TreeNode | null): number[] {
+    if (node === null) {
+        return [];
+    }
+
+    return [
+        ...inOrderTraversal(node.left),
+        node.val,
+        ...inOrderTraversal(node.right)
+    ];
+}
+
+function kthSmallest(root: TreeNode | null, k: number): number {
+    const result = inOrderTraversal(root);
+    return result[k - 1]
+}

number-of-1-bits/gitsunmin.ts

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+/**
+ * https://leetcode.com/problems/number-of-1-bits/
+ * time complexity : O(log n)
+ * space complexity : O(log n)
+ */
+function hammingWeight(n: number): number {
+    return n.toString(2).replaceAll('0', '').length
+}

palindromic-substrings/gitsunmin.ts

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+/**
+ * https://leetcode.com/problems/palindromic-substrings/
+ * time complexity : O(n^2)
+ * space complexity : O(n)
+ */
+
+const expandAroundCenter = (s: string, left: number, right: number): number => {
+    if (left < 0 || right >= s.length || s[left] !== s[right]) {
+        return 0;
+    }
+    return 1 + expandAroundCenter(s, left - 1, right + 1);
+};
+
+function countSubstrings(s: string): number {
+
+    return [...Array(s.length).keys()].reduce((totalCount, i) => {
+        return totalCount +
+            expandAroundCenter(s, i, i) +
+            expandAroundCenter(s, i, i + 1);
+    }, 0);
+};

top-k-frequent-elements/gitsunmin.ts

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+/**
+ * https://leetcode.com/problems/top-k-frequent-elements/
+ * time complexity : O(n)
+ * space complexity : O(n)
+ */
+function topKFrequent(nums: number[], k: number): number[] {
+    const record = nums.reduce((acc, curr) => {
+        acc[curr] = (acc[curr] ?? 0) + 1;
+        return acc;
+    }, {});
+
+    const array: Array<number[]> = new Array(nums.length);
+    for (const num in record) {
+        const v = record[num];
+        if (!array[v]) {
+            array[v] = [];
+        }
+        array[v].push(Number(num));
+    }
+
+    return array.reduce((acc, curr) => [...curr, ...acc], []).slice(0, k);
+};