Pop Sequence（java实现）

7-3 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 $N$ 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

思路：

1、主要考察栈（stack）的使用，我直接用了java中stack这个类，当然也可以自己写一个类似的类。

2、一开始我想直接根据M,N,K把所有可能的结果序列都存贮起来，用测试数据逐个对比，后来发现工作量太大，放弃。还是采用给定的输出序列，用人工的想法逐个判断。

3、有一些坑，注意栈的容量K比N小的情况，若出现stack.size( )大于N，直接bool=false,break。

4、犯了一个很傻的错误，ArrayList<int []>存贮的其实是数组的引用，而不会开辟空间存储数组中的具体元素。

5、简单的流程图：

import java.util.ArrayList;
import java.util.Scanner;
import java.util.Stack;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner in = new Scanner(System.in);
		int m = in.nextInt();
		int n = in.nextInt();
		int k = in .nextInt();
		
		ArrayList<int []> al = new ArrayList<>();
		
		int [ ][] a = new int [k][n];
		for(int i= 0;i<k;i++) {
			for(int j = 0;j<n;j++) {
				a[i][j] = in.nextInt();
			}
			al.add(a[i]);
		}
		judge(al,k,n,m);
	}

	private static void judge(ArrayList<int[]> al, int k, int n,int size) {
		// TODO Auto-generated method stub
		Stack<Integer> st = new Stack<>();
		int temp = 0 ;
		boolean bool = true;
		for(int i = 0;i<k;i++) {
			int [] a = al.get(i);
			for(int j =0; j < n; j++) {
				int m = a[j];
				if( m>temp) {
					bool=caozuo(st,m,temp,size);
					temp=m;
					if(bool==false)
						break;
				}
	/////////////////////////////////////////////			
				else if(m<temp) {
						if(!st.empty()) {
						int pop = st.pop();
						if(m==pop) {
						
							continue;
						}
						else {
							bool = false;
							break;
						}
					}
						else
							bool = false;
				}
	//////////////////////////////////////////////////////
			}
			
			if(bool == true)
				System.out.println("YES");
			else
				System.out.println("NO");
			bool=true;
			temp=0;
			st.clear();
		}
	}

	private static boolean caozuo(Stack<Integer>st,int  m, int temp,int size) {
		// TODO Auto-generated method stub
		int k = m-temp;
		boolean bool = true;
		for(int i=1;i<=k;i++) {
			st.push(temp+i);
			if(st.size()>size) {
				bool=false;
				break;
			}
		}
	
			st.pop();

		return bool;
	}

}