solution 3sum,valid-binary-search-tree

Author: HISEHOONAN

3sum/HISEHOONAN.swift

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+//
+//  241.swift
+//  Algorithm
+//
+//  Created by 안세훈 on 4/8/25.
+//
+
+//3Sum
+class Solution { //정렬 + two pointer
+    func threeSum(_ nums: [Int]) -> [[Int]] {
+        let nums = nums.sorted() //배열을 오름차순으로 정렬
+        var result: [[Int]] = [] // 결과를 저장할 배열.
+
+        for i in 0..<nums.count { // i를 0번째 배열부터 순회.
+            if i > 0 && nums[i] == nums[i - 1] {
+                continue
+            }
+
+            var left = i + 1 //left는 i+1번째 인덱스
+            var right = nums.count - 1 //right는 배열의 끝번째 인덱스
+
+            while left < right {
+                let sum = nums[i] + nums[left] + nums[right]
+
+                if sum == 0 {
+                    result.append([nums[i], nums[left], nums[right]])
+
+                    // 중복 제거
+                    while left < right && nums[left] == nums[left + 1] {
+                        left += 1
+                    }
+                    while left < right && nums[right] == nums[right - 1] {
+                        right -= 1
+                    }
+
+                    left += 1
+                    right -= 1
+                } else if sum < 0 {
+                    left += 1
+                } else {
+                    right -= 1
+                }
+            }
+        }
+
+        return result
+    }
+}

validate-binary-search-tree/HISEHOONAN.swift

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+//
+//  251.swift
+//  Algorithm
+//
+//  Created by 안세훈 on 4/8/25.
+//
+
+//Validate Binary Search Tree
+
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init() { self.val = 0; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
+ *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
+ *         self.val = val
+ *         self.left = left
+ *         self.right = right
+ *     }
+ * }
+ */
+
+class Solution {
+    func isValidBST(_ root: TreeNode?) -> Bool {
+        return validate(root, min: nil, max: nil)
+    }
+
+    private func validate(_ node: TreeNode?, min: Int?, max: Int?) -> Bool {
+        guard let node = node else { return true }
+
+        if let min = min, node.val <= min { return false }
+        if let max = max, node.val >= max { return false }
+
+        return validate(node.left, min: min, max: node.val) &&
+               validate(node.right, min: node.val, max: max)
+    }
+}