[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.

思路：中序遍历二叉树，如果序列不是递增的话则返回false；

代码：

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    bool isValidBST(TreeNode *root) {        if(root==NULL) return true;        vector<int> res;        res.clear();        inorder(root,res);        int len=res.size();        for(int i=1;i<len;i++){            if(res[i]<=res[i-1]) return false;        }        return true;    }    void inorder(TreeNode *root,vector<int> &res){        if(root==NULL) return;        inorder(root->left,res);        res.push_back(root->val);        inorder(root->right,res);    }};