LeetCode算法题目：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.

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分析：

根据搜索二叉树的特点，对其进行中序遍历并将值存入vector中，则得到的应该是一个有序的序列。故先用中序遍历记下所有节点，然后判断此数组是否是已排序的。

代码：

/** * 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:    void getval(TreeNode *root,vector<int> &result)//中序遍历二叉树并存储其值    {        if(root->left!=NULL)        {            getval(root->left,result);        }        result.push_back(root->val);        if(root->right!=NULL)        {            getval(root->right,result);        }        return;    }    bool isValidBST(TreeNode *root) {        if(root==NULL)return true;        vector<int> result;        getval(root,result);        for(int i=1;i<result.size();i++)//判断所得的值是否有序        {            if(result[i-1]>=result[i])            {                return false;            }        }        return true;    }};