feat: Validate Binary Search Tree

Author: gwbaik9717

validate-binary-search-tree/gwbaik9717.js

@@ -0,0 +1,71 @@
+// Time complexity: O(n)
+// Space complexity: O(n)
+
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val, left, right) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.left = (left===undefined ? null : left)
+ *     this.right = (right===undefined ? null : right)
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @return {boolean}
+ */
+var isValidBST = function (root) {
+  const dfs = (root) => {
+    let minVal = root.val;
+    let maxVal = root.val;
+
+    if (root.left) {
+      if (root.left.val >= root.val) {
+        return false;
+      }
+
+      const result = dfs(root.left);
+
+      if (!result) {
+        return false;
+      }
+
+      const [min, max] = result;
+
+      if (max >= root.val) {
+        return false;
+      }
+
+      minVal = Math.min(minVal, min);
+      maxVal = Math.max(maxVal, max);
+    }
+
+    if (root.right) {
+      if (root.right.val <= root.val) {
+        return false;
+      }
+
+      const result = dfs(root.right);
+
+      if (!result) {
+        return false;
+      }
+
+      const [min, max] = result;
+
+      if (min <= root.val) {
+        return false;
+      }
+
+      minVal = Math.min(minVal, min);
+      maxVal = Math.max(maxVal, max);
+    }
+
+    return [minVal, maxVal];
+  };
+
+  if (dfs(root)) {
+    return true;
+  }
+
+  return false;
+};