validate-binary-search-tree

题目：

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.

OJ’s Binary Tree Serialization:
The serialization of a binary tree follows a level order traversal, where ‘#’ signifies a path terminator where no node exists below.
Here’s an example:
1
/ \
2 3
/
4
\
5
The above binary tree is serialized as”{1,2,3,#,#,4,#,#,5}”.

程序：

class Solution {
public:
    bool isValidBST(TreeNode *root) {
        return solve(root,INT_MIN,INT_MAX);
    }
    bool solve(TreeNode *root,int l,int h)
    {
        if(root == NULL)
            return true;
        if(root->val <= l || root->val >= h)
            return false;
        return (solve(root->left,l,root->val) && solve(root->right,root->val,h));
    }
};