/**
 * 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) {
	    return isValidBST(root, nullptr, nullptr);
     }

     bool isValidBST(TreeNode* root, TreeNode* minNode, TreeNode* maxNode) {
         //此节点不存在，返回true
	    if (!root) return true;
         //此结点比最大值要大或者比最小值要小.
	    if (minNode && root->val <= minNode->val || maxNode && root->val >= maxNode->val)
		    return false;
         //继续判断左边或者右边，判断左边的时候传入最大值为root,判断右边的时候传入最小值为root
	    return isValidBST(root->left, minNode, root) && isValidBST(root->right, root, maxNode);
     }
};