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.

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

//还好对于int32位长整形，涉及溢出问题时，我们还可以用long来覆盖。
//如果函数形参是long的话，恐怕就得用额外一个标志位表示溢出了。
/**
 * 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 == NULL){
            return true;
        }
        else{
            return _isValidBST(root->left, LONG_MIN, root->val) && _isValidBST(root->right, (long)root->val + 1, LONG_MAX);
        }
    }
    bool _isValidBST(TreeNode * root, long min, long max){
        if(root == NULL){
            return true;
        }
        if(root->val < min || root->val >= max){
            return false;
        }
        else{
            return _isValidBST(root->left, min, root->val) && _isValidBST(root->right, (long)root->val + 1, max);
        }
    }
};