1051 Pop Sequence （25 分)

2019-03-27

1051 Pop Sequence （25 分)

Given a stack which can keep M numbers at most. Push N numbers in the order of 1, 2, 3, …, N and pop randomly. You are supposed to tell if a given sequence of numbers is a possible pop sequence of the stack. For example, if M is 5 and N is 7, we can obtain 1, 2, 3, 4, 5, 6, 7 from the stack, but not 3, 2, 1, 7, 5, 6, 4.

Input Specification:

Each input file contains one test case. For each case, the first line contains 3 numbers (all no more than 1000): M (the maximum capacity of the stack), N (the length of push sequence), and K (the number of pop sequences to be checked). Then K lines follow, each contains a pop sequence of Nnumbers. All the numbers in a line are separated by a space.

Output Specification:

For each pop sequence, print in one line “YES” if it is indeed a possible pop sequence of the stack, or “NO” if not.

Sample Input:

5 7 5
1 2 3 4 5 6 7
3 2 1 7 5 6 4
7 6 5 4 3 2 1
5 6 4 3 7 2 1
1 7 6 5 4 3 2

Sample Output:

YES
NO
NO
YES
NO

作者: CHEN, Yue

单位: 浙江大学

时间限制: 400 ms

内存限制: 64 MB

代码长度限制: 16 KB

题目大意

给出一个栈的大小M，数字N，1-N按顺序入栈。再给出一组数字，问是否存在相应的出栈序列。

分析

利用vec存放出栈序列，stack作为栈。每次比较栈顶元素是否与vec当前元素相等，不相等则入栈，判断stack中大小是否超过M，超过输出NO。相等则stack出栈并前进vec。vec遍历完后若stack为空则输出YES。

代码

#include <iostream>
#include <vector>
#include <algorithm>
#include <deque>
#include <map>
#include <set>
#include <cstring>
#include <cmath>
#include<sstream>

using namespace std;

int main() {
    int m,n,k;

    vector<int> stack;
    vector<int> vec;

    cin>>m>>n>>k;

    for(int i=0;i<k;i++){
        int flag=0;
        int a=1;
        vec.clear();
        stack.clear();
        vec.resize(n);
        for(int j=0;j<n;j++)
            cin>>vec[j];

        for(int i=0;i<n;i++) {
            if(stack.empty())
                stack.push_back(a++);
            while (stack.back() != vec[i]) {
                stack.push_back(a++);
                if(stack.size()>m)
                {
                    cout<<"NO"<<endl;
                    flag=1;
                    break;
                }
            }
            stack.pop_back();
            if(flag)
                break;
        }
        if(flag)
            continue;
        if(stack.empty())
            cout<<"YES"<<endl;
    }
    return 0;
}