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.
Example 1:

    2   / \  1   3

Binary tree [2,1,3], return true.
Example 2:

    1   / \  2   3

Binary tree [1,2,3], return false.

思路：中序遍历。将二叉树中序遍历后存放到数组中，判断数组是升序的，就是二叉搜索树。

/** * 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) return true;        vector<int>vals;        inorder(root,vals);        for(int i=0;i<vals.size()-1;i++){            if(vals[i]>=vals[i+1]) return false;        }        return true;    }    void inorder(TreeNode* root,vector<int>&vals){        if(root){            inorder(root->left,vals);            vals.push_back(root->val);            inorder(root->right,vals);        }    }};