solve: Week 02 validate binary search tree

Author: eunice-hong

validate-binary-search-tree/eunice-hong.ts

@@ -0,0 +1,29 @@
+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 {
+  // Helper function to check if a tree is a valid binary search tree
+  function isValidTree(node: TreeNode | null, min: number | null, max: number | null): boolean {
+    // If the node is null, the tree is valid
+    if (node === null) return true;
+
+    // If the node's value is less than the minimum or greater than the maximum, the tree is not valid
+    if ((min !== null && node.val <= min) || (max !== null && node.val >= max)) {
+      return false;
+    }
+
+    // Recursively check the left and right subtrees
+    return isValidTree(node.left, min, node.val) && isValidTree(node.right, node.val, max);
+  }
+
+  // Check if the tree is a valid binary search tree
+  return isValidTree(root, null, null);
+}