[sukyoungshin] WEEK 02

3sum/sukyoungshin.ts

@@ -0,0 +1,36 @@
+function threeSum(nums: number[]): number[][] {
+  nums.sort((a, b) => a - b);
+  const result: number[][] = [];
+  const target = 0;
+
+  for (let i = 0; i < nums.length - 2; i++) {
+    // 중복된 시작 숫자는 건너뛰기
+    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 === target) {
+        result.push([nums[i], nums[left], nums[right]]);
+
+        // 중복된 left 값 건너뛰기
+        while (left < right && nums[left] === nums[left + 1]) left++;
+        // 중복된 right 값 건너뛰기
+        while (left < right && nums[right] === nums[right - 1]) right--;
+
+        // 다음 조합 시도
+        left++;
+        right--;
+      } else if (sum < 0) {
+        left++;
+      } else {
+        right--;
+      }
+    }
+  }
+
+  return result;
+};

climbing-stairs/sukyoungshin.ts

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+// 1번풀이 (hashMap)
+function climbStairs1(n: number): number {
+  const dp: Record<number, number> = {
+    1: 1,
+    2: 2,
+  };
+
+  for (let i = 3; i <= n; i++) {
+    dp[i] = dp[i - 1] + dp[i - 2];
+  }
+
+  return dp[n];
+};
+
+// 2번풀이 (배열)
+function climbStairs2(n: number): number {
+  const dp = [0, 1, 2]; // dp[1] = 1, dp[2] = 2
+
+  for (let i = 3; i <= n; i++) {
+    dp[i] = dp[i - 1] + dp[i - 2];
+  }
+
+  return dp[n];
+};
+
+// 3번풀이
+// 불필요한 배열 전체 저장을 제거하고, 필요한 값만 2개만 유지
+function climbStairs(n: number): number {
+  if (n <= 2) return n;
+
+  let prev1 = 1; // dp[i - 2]
+  let prev2 = 2; // dp[i - 1]
+
+  for (let i = 3; i <= n; i++) {
+    const current = prev1 + prev2;
+    prev1 = prev2;
+    prev2 = current;
+  }
+
+  return prev2;
+};
+

product-of-array-except-self/sukyoungshin.ts

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+// 1번 풀이
+function productExceptSelf1(nums: number[]): number[] {
+  const result: number[] = new Array(nums.length).fill(1);
+
+  applyLeftProducts(nums, result);
+  applyRightProducts(nums, result);
+
+  return result;
+};
+
+function applyLeftProducts(nums: number[], result: number[]) {
+  let prefix = 1;
+  for (let i = 0; i < nums.length; i++) {
+    result[i] = prefix;
+    prefix *= nums[i];
+  }
+};
+
+function applyRightProducts(nums: number[], result: number[]) {
+  let suffix = 1;
+  for (let i = nums.length - 1; i >= 0; i--) {
+    result[i] *= suffix;
+    suffix *= nums[i];
+  }
+};
+
+// 2번 풀이
+function productExceptSelf2(nums: number[]): number[] {
+  const length = nums.length;
+  const result: number[] = new Array(length).fill(1);
+
+  // 1단계: 왼쪽 누적 곱 저장
+  let prefix = 1;
+  for (let i = 0; i < length; i++) {
+    result[i] = prefix;
+    prefix *= nums[i];
+  }
+
+  // 2단계: 오른쪽 누적 곱 곱하기 (in-place)
+  let suffix = 1;
+  for (let i = length - 1; i >= 0; i--) {
+    result[i] *= suffix;
+    suffix *= nums[i];
+  }
+
+  return result;
+};

valid-anagram/sukyoungshin.ts

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+// 1번풀이 (hashTable)
+function isAnagram1(s: string, t: string): boolean {
+  if (s.length !== t.length) return false;
+
+  const charCount: Record<string, number> = {};
+
+  for (const char of s) {
+    charCount[char] = (charCount[char] ?? 0) + 1;
+  }
+
+  for (const char of t) {
+    if (!charCount[char]) return false;
+    charCount[char]--;
+  }
+
+  return true;
+};
+
+// 2번풀이 (sort)
+function isAnagram2(s: string, t: string): boolean {
+  if (s.length !== t.length) return false;
+
+  const sortedS = s.split('').sort().join('');
+  const sortedT = t.split('').sort().join('');
+
+  return sortedS === sortedT;
+};
+

validate-binary-search-tree/sukyoungshin.ts

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+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;
+  }
+}
+
+// 1번풀이
+function isValidBST1(root: TreeNode | null): boolean {
+  const values: number[] = [];
+
+  // 중위노드 : 왼쪽 자식 -> 부모 노드 -> 오른쪽 자식
+  function inorder(node: TreeNode | null) {
+    if (!node) return null;
+    const { val, left, right } = node;
+
+    inorder(left);
+    values.push(val);
+    inorder(right);
+  }
+
+  inorder(root);
+
+  for (let i = 1; i < values.length; i++) {
+    if (values[i] <= values[i - 1]) return false;
+  }
+
+  return true;
+};
+
+// 2번풀이 (배열 대신 변수 사용)
+function isValidBST2(root: TreeNode | null): boolean {
+  let prev: number | null = null;
+  let isValid = true;
+
+  function inorder(node: TreeNode | null) {
+    if (!node || !isValid) return;
+
+    inorder(node.left);
+
+    if (prev !== null && node.val <= prev) {
+      isValid = false;
+      return;
+    }
+    prev = node.val;
+
+    inorder(node.right);
+  }
+
+  inorder(root);
+  return isValid;
+};