Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

题解

very special problem, the key is to distiguished the "left" and "right" nodes, and under what circumstances, return TRUE

if(root==null) return true;

 

 public boolean isValidBST(TreeNode root) {
        return helper(root,null,null);
    }
    private boolean helper(TreeNode root, TreeNode left, TreeNode right){
        if(root==null) return true;
        if(left!=null && left.val>=root.val) return false;
        if(right!=null && right.val<=root.val) return false;
        return helper(root.left, left, root) && helper(root.right, root, right);
    }