用一个数组A[1..n]来实现两个栈

#include <stdio.h>
#include <stdlib.h>

#define MAXNUM 4
#define OVERFLOW -1
#define UNDERFLOW -2
#define FLAGERROR -3

typedef int elemtype;

typedef struct stack
{
        int top;
        int flag;
}stack;

typedef struct ShareArrStack
{
        elemtype data[MAXNUM];
        stack stack1;
        stack stack2;
}ShareArrStack;

void InitStack(ShareArrStack *s)
{
        (s->stack1).top = -1;
        (s->stack1).flag = 1;
        (s->stack2).top = MAXNUM;
        (s->stack2).flag = 2;
}

int IsFull(ShareArrStack *s)
{
        return (s->stack1.top + 1 == s->stack2.top);
}

int IsEmpty(stack *sn)
{
        return ((sn->top == -1)||(sn->top == MAXNUM));
}

void Push(ShareArrStack *s,stack sn,elemtype x)
{
        if (IsFull(s))
        {
                printf("Stack overflow!");
                exit(OVERFLOW);
        }
        else
        {
                switch(sn.flag)
                {
                case 1:
                        {
                                s->stack1.top += 1;
                                s->data[s->stack1.top] = x;
                                break;
                        }
                case 2:
                        {
                                s->stack2.top -= 1;
                                s->data[s->stack2.top] = x;
                                break;
                        }
                default:
                        {
                                exit(FLAGERROR);
                        }
                }
        }
}

elemtype Pop(ShareArrStack *s,stack sn)
{
        if (IsEmpty(&sn))
        {
                printf("stack underflow!");
                exit(UNDERFLOW);
        }
        else
        {
                switch(sn.flag)
                {
                case 1:
                        {
                                return (s->data[s->stack1.top --]);
                                break;
                        }
                case 2:
                        {
                                return (s->data[s->stack2.top ++]);
                                break;
                        }
                default:
                        {
                                exit(FLAGERROR);
                        }
                }
        }
}

int main()
{
        ShareArrStack *s = NULL;
        s = (ShareArrStack *)malloc(sizeof(ShareArrStack));
        InitStack(s);
        Push(s,s->stack1,1);
        Push(s,s->stack2,9);
        Push(s,s->stack1,2);
        Push(s,s->stack2,8);
        printf("%d, ",Pop(s,s->stack2));
        printf("%d",Pop(s,s->stack1));
        return 0;
}

[root@localhost C++]# ./Arr2Stack
8, 2[root@localhost C++]#