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;

    }
};