7-3 Pop Sequence (25 分)（C source code）

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

结尾无空行

Sample Output:

YES
NO
NO
YES
NO

结尾无空行

题目理解：

考察的是堆栈的知识，LIFO(后进先出)。给出了一个1，2，... ，n的顺序序列，然后与要被check的序列比较，看顺序序列是否能通过大小为m的堆栈实现被check的序列。我的思路就是模拟堆栈的过程，首先创建一个m的堆栈，然后从check的序列的第一个元素开始，进行比较。

case 1. 栈为空or栈的顶部元素==check序列中被指向的元素。

此时，栈顶元素pop出去，指向下一个check元素

case 2.栈的顶部元素！=check序列中被指向的元素

1）此时，如果栈已经满了，那么就证明没办法实现

2）栈未满，栈push进下一个元素

为了图省事，把被check的序列和栈写成了一个struct~

答案（AC）

#include 
#include 

struct Stack
{
    int *array;
    int maxsize;
    int top;
    int cnt;
};
typedef struct Stack *stack;
stack Createstack(int n);
void ReadStack(stack S);
void pop(stack S);
void push(stack S);
int Isfull(stack S);
void RenewStack(stack S);
int main()
{
    int n, m, k;
    int i, j;
    scanf("%d %d %d", &m, &n, &k);
    
    stack S1 = Createstack(m);
    stack S2 = Createstack(n); //S2 is to be checked
    for (i = 0; i < k; i++)
    {
        ReadStack(S2);
        while (S2->top < n)
        {
            if (S1->top >= 0 && S1->array[S1->top] == S2->array[S2->top])
            {
                S2->top++;
                pop(S1);
            }
            else
            {
                if (Isfull(S1))
                    break;
                else
                    push(S1);
            }
        }
        if (S2->top == n)
            printf("YESn");
        else
            printf("NOn");
        RenewStack(S1);
    }
    return 0;
}
stack Createstack(int n)
{
    int i;
    stack S = (stack)malloc(sizeof(struct Stack));
    S->array = (int *)malloc(sizeof(int) * n);
    S->maxsize = n;
    S->cnt = 1;
    S->top = -1;
    return S;
}
void ReadStack(stack S)
{
    int i;
    for (i = 0; i < S->maxsize; i++)
    {
        scanf("%d", &S->array[i]);
    }
    S->top = 0;
}
void pop(stack S)
{
    S->top--;
}
void push(stack S)
{
    S->array[++S->top] = S->cnt++;
}
int Isfull(stack S)
{
    if (S->top == S->maxsize - 1)
        return 1;
    else
        return 0;
}
void RenewStack(stack S){
    S->top=-1;
    S->cnt=1;
}