【Leetcode】Valid Binary Search Tree

【题目】

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

【代码】

/**
* 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