PAT甲级——A1051 Pop Sequence

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 N numbers. 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

#include <iostream>
#include <stack>
using namespace std;
int M, N, K;
int main()
{
    cin >> M >> N >> K;    
    for (int i = 0; i < K; ++i)
    {
        stack<int>s;
        int a, b = 1, flag = 1;
        for (int j = 0; j < N; ++j)
        {
            cin >> a;
            while (s.size() < M && (s.empty() || s.top() != a))
                s.push(b++);
            if (s.top() == a)
                s.pop();
            else
                flag = 0;
        }
        if (flag == 1)
            cout << "Yes" << endl;
        else
            cout << "No" << endl;
    }
    return 0;
}