add Validate Binary Search Tree solution

Author: HoonDongKang

validate-binary-search-tree/HoonDongKang.ts

@@ -0,0 +1,50 @@
+/**
+ * [Problem]: [98] Validate Binary Search Tree
+ *
+ * (https://leetcode.com/problems/validate-binary-search-tree/description/)
+ */
+
+/**
+ * Definition for a binary tree node.
+ * 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)
+ *     }
+ * }
+ */
+
+function isValidBST(root: TreeNode | null): boolean {
+    //시간 복잡도 O(n)
+    //공간 복잡도 O(n)
+    function dfs(node: TreeNode | null, low: number | null, high: number | null) {
+        if (!node) return true;
+        if ((low !== null && node.val <= low) || (high !== null && node.val >= high)) {
+            return false;
+        }
+
+        return dfs(node.left, low, node.val) && dfs(node.right, node.val, high);
+    }
+
+    //시간 복잡도 O(n)
+    //공간 복잡도 O(n)
+    let prev: number | null = null;
+    function inOrder(node: TreeNode | null): boolean {
+        if (!node) return true;
+
+        if (!inOrder(node.left)) return false;
+
+        if (prev !== null && node.val <= prev) return false;
+
+        prev = node.val;
+
+        return inOrder(node.right);
+    }
+
+    // return dfs(root, null, null);
+    return inOrder(root);
+}