Validate Binary Search Tree

题目大意： 判断一个二叉树是否是个排序二叉树，即根节点大于左节点，小于右节点，同时它的左右子树也满足这个要求

解题思路：中序遍历，如果当前节点小于等于左子树的最大节点，则不符合要求，若当前节点大于等于右子树的最小节点，则不符合要求

/** * 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) {        int minVal = INT_MAX, maxVal = INT_MIN;        return isValidBSTAssist(root, minVal, maxVal);    }private:    bool isValidBSTAssist(TreeNode *root, int &minVal, int &maxVal) {        if(root == NULL) {            return true;        }        int left_min, left_max;        if(!isValidBSTAssist(root->left, left_min, left_max)) {            return false;        }        if(root->left != NULL) {            if(root->val <= left_max) {                return false;            }        } else {            left_min = left_max = root->val;        }        int right_min, right_max;        if(!isValidBSTAssist(root->right, right_min, right_max)) {            return false;        }        if(root->right != NULL) {            if(root->val >= right_min) {                return false;            }        } else {            right_min = right_max = root->val;        }        minVal = min(root->val, min(left_min, right_min));        maxVal = max(root->val, max(left_max, right_max));        return true;    }};