leetcode: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.

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"{1,#,2,3}"

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题目地址：https://oj.leetcode.com/problems/validate-binary-search-tree/
解题思路：找出二叉树的中序遍历，如果出现乱序的地方就不是二叉查找树。

代码：

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode *pre;
bool ans;
bool isValidBST(TreeNode *root) {
//中序遍历，如果得到不是顺序的地方，返回false
if(root==NULL) return true;
if(root->left==NULL&&root->right==NULL) return true;

pre=NULL;
ans=true;
inorder(root);
return ans;

}
private:
void inorder(TreeNode *root){
if(root==NULL) return;
inorder(root->left);
if(pre==NULL){
pre=root;
}
else{
if(pre->val>=root->val){
ans=false;
//找到了不是顺序的地方就直接返回，不用再继续遍历
return;
}
pre=root;
}
inorder(root->right);
}
};