LeetCode Validate Binary Search Tree

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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.

思路：中序遍历的话就是一个递增数组，所以可以利用这点去中序遍历判断。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private TreeNode pre = null;
	
	public boolean isValidBST(TreeNode root) {  
		if (root == null) return true;
		
		if (!isValidBST(root.left)) return false;
		if (pre != null && root.val <= pre.val) return false;
		pre = root;
		return isValidBST(root.right);
	}
}

LeetCode Validate Binary Search Tree

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原文地址：http://blog.csdn.net/u011345136/article/details/45270129