LeetCode 98. Validate Binary Search Tree

98. 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.

题目大意：给一个二叉树，判断这个二叉树是不是合法的二叉搜索树～
分析：既然是二叉搜索树，那么按照左根右遍历后的结果一定是增序～所以只需要中序遍历一遍，判断遍历结果的数组是不是后面数一定大于前面数就可以了～

class Solution {private:    vector<int> v;public:    bool isValidBST(TreeNode* root) {        if(root == NULL ||(root->left == NULL && root->right == NULL)) return true;        inorder(root);        for(int i = 1; i < v.size(); i++)            if(v[i] <= v[i-1]) return false;        return true;    }    void inorder(TreeNode* root) {        if(root == NULL) return;        inorder(root->left);        v.push_back(root->val);        inorder(root->right);    }};