【Leetcode】Valid Binary Search Tree

【題目】

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.

【思路】

查看clean book

【代碼】

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        
        return valid(root, null,null);
        }
        
        public boolean valid(TreeNode root, Integer low, Integer high){
            if(root == null) return true;
            return (low == null || root.val > low) && (high == null || root.val < high) &&
            valid(root.left,low,root.val) && valid(root.right,root.val,high);
        }
}

DFS