leetcode--Validate Binary Search Tree

1.题目描述

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.

2.解法分析

对树进行先序遍历，如果先序遍历的过程中始终有前一个元素比后一个元素小，那么就是一棵有效的BST，反之则不是

/** * Definition for binary tree * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: bool isValidBST(TreeNode *root) { // Start typing your C/C++ solution below // DO NOT write int main() function if(!root)return true; vector<TreeNode *> v; int cur; int prev; bool firstRound=true; TreeNode *curT=root; while(curT||!v.empty()) { while(curT) { v.push_back(curT); curT=curT->left; } if(!v.empty()) { curT=v.back();v.pop_back(); if(firstRound){cur=curT->val;prev=cur;firstRound=false;} else { cur=curT->val; if(cur<=prev)return false; prev=cur; }   curT=curT->right; } } return true; } };