Leetcode-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.

Example 1:

    2
   / \
  1   3

Binary tree  [2,1,3], return true.

Example 2:

    1
   / \
  2   3

Binary tree  [1,2,3], return false.

思路：与之前的BST题类似，如果用中序遍历去访问一棵BST，那么得到的是一个有序递增的序列，因此可以考虑用两个相邻节点（pre和node）不断地用中序遍历访问二叉树，若出现node比pre还小的情况，则必定不是BST。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isValidBST(TreeNode* root) {
        TreeNode* pre = NULL;
        return validate(root,pre);
    }
    bool validate(TreeNode* node,TreeNode* &pre){
        if(node==NULL)
            return true;
        if(!validate(node->left,pre))
            return false;
        if(pre!=NULL && pre->val >= node->val)
            return false;
        pre = node;
        return validate(node->right,pre);
    }
};