ValidateBinarySearchTree98

Author: fluency03

src/ValidateBinarySearchTree98.java

@@ -38,20 +38,45 @@ public boolean isValidBST(TreeNode root) {
 
     private boolean helper(TreeNode root, long min, long max) {
         if (root == null) return true;
+        if (root.val <= min || root.val >= max) return false;
+        return helper(root.left, min, root.val) && helper(root.right, root.val, max);
+    }
 
-        if (root.val <= min || root.val >= max) {
-            return false;
-        }
 
-        return helper(root.left, min, root.val) && helper(root.right, root.val, max);
+    public boolean isValidBST2(TreeNode root) {
+        if (root == null) return true;
+        return isValidBST(root, new int[]{0, 0});
     }
 
+    private boolean isValidBST(TreeNode root, int[] bounds) {
+        bounds[0] = root.val;
+        bounds[1] = root.val;
+        if (root.left == null && root.right == null) {
+            return true;
+        }
+
+        if (root.left != null) {
+            int[] leftBounds = new int[]{0, 0};
+            boolean left = isValidBST(root.left, leftBounds);
+            if (!left || leftBounds[1] >= root.val) return false;
+            bounds[0] = leftBounds[0];
+        }
+
+        if (root.right != null) {
+            int[] rightBounds = new int[]{0, 0};
+            boolean right = isValidBST(root.right, rightBounds);
+            if (!right || rightBounds[0] <= root.val) return false;
+            bounds[1] = rightBounds[1];
+        }
+
+        return true;
+    }
 
 
     /**
      * https://discuss.leetcode.com/topic/46016/learn-one-iterative-inorder-traversal-apply-it-to-multiple-tree-questions-java-solution
      */
-    public boolean isValidBST2(TreeNode root) {
+    public boolean isValidBST3(TreeNode root) {
         if (root == null) return true;
         Stack<TreeNode> stack = new Stack<>();
         TreeNode pre = null;