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   3Binary tree [2,1,3], return true.Example 2:    1   / \  2   3Binary tree [1,2,3], return false.思路：BST的中序遍历的升序列*//** * 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) {        if(!root) return true;        vector<int> v;        inorder(root,v);        for(int i=0;i<v.size()-1;i++)            if(v[i]>=v[i+1]) return false;        return true;    }    void inorder(TreeNode*root,vector<int>& v)    {        if(!root) return;        inorder(root->left,v);        v.push_back(root->val);        inorder(root->right,v);    }    bool isValidBST(TreeNode* root) {        long preVal=LONG_MIN;        return inorder_chech(root,preVal);    }    bool inorder_chech(TreeNode* root,long &preVal)    {        if(!root) return true;        if(!inorder_chech(root->left,preVal)) return false;        if(root->val <= preVal) return false;        preVal=root->val;        if(!inorder_chech(root->right,preVal)) return false;        return true;    }};