Validate Binary Search Tree问题及解法

问题描述：

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.

示例:

    2   / \  1   3

Binary tree [2,1,3], return true.

    1   / \  2   3

Binary tree [1,2,3], return false.

问题分析：

根据二叉搜索树而言，深度优先遍历后得到一个递增的数组。到时验证这个数组的合理性就能判断树是否为BST了。

过程详见代码：

/** * 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:    bool isValidBST(TreeNode* root) {vector<int> res;bl(root, res);for (int i = 1; i < res.size(); i++){if (res[i - 1] >= res[i]) return false;}return true;}void bl(TreeNode* root,vector<int> &res){if (root == NULL) return;bl(root->left, res);res.push_back(root->val);bl(root->right, res);}};