98. 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.

 

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

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/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
       
       return helper(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
        
    }
    
    private boolean helper(TreeNode node, int l, int r){
        
        if(node == null)    return true;
        
        if(node.val <=l || node.val >= r)
            return false;
            
        return helper(node.left, l, node.val) && helper(node.right, node.val, r);
        
    }
}