学步园

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.

比较简单，中序遍历，并保持上一个被遍历的值与当前值比较就行了。

代码如下：

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
       if ( !root )
            return true;
        pre=-1000000;
        return check(root);
    }
    bool check(TreeNode* root)
    {
        if ( !root )
            return true;
        if ( check(root->left) )
        {
            if ( root->val <= pre)
                return false;
            pre=root->val;
            return check(root->right);
        }
        else
            return false;
            
    }
    int pre;
};