第十八周：[Leetcode]98. 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.

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判断是否为有效二叉查找树，用中序遍历二叉树，判断遍历序列是否为升序数组即可。

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class Solution {public:    vector<int> midorder;    bool isValidBST(TreeNode* root) {        if(root == NULL)            return true;        midorder.clear();        dfs(root);        for(int i = 0;i < midorder.size() - 1;i++)            if(midorder[i] >= midorder[i + 1])                return false;        return true;    }    void dfs(TreeNode* root) {        if(root == NULL)            return;        dfs(root -> left);        midorder.push_back(root -> val);        dfs(root -> right);    }};