Validate Binary Search Tree
给一个二叉树, 问是不是bst. 就利用bst的性质验证.
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Solution {
double lastval = Long.MIN_VALUE;
public boolean isValidBST(TreeNode root) {
if(root == null)
return true;
if(!isValidBST(root.left))
return false;
if(lastval>=root.val)
return false;
lastval = root.val;
if(!isValidBST(root.right))
return false;
return true;
}
}