程序代码：

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

#define STACK_INIT_SIZE 20
#define STACKINCREMENT  10

typedef double ElemType;
typedef struct
{
    ElemType *base;
    ElemType *top;
    int stackSize;
}sqStack;

InitStack(sqStack *s)
{
    s->base = (ElemType *)malloc(STACK_INIT_SIZE * sizeof(ElemType));
    if( !s->base )
        exit(0);

    s->top = s->base;
    s->stackSize = STACK_INIT_SIZE;
}

Push(sqStack *s, ElemType e)
{
    // 栈满，追加空间，鱼油必须懂！
    if( s->top - s->base >= s->stackSize )
    {
        s->base = (ElemType *)realloc(s->base, (s->stackSize + STACKINCREMENT) * sizeof(ElemType));
        if( !s->base )
            exit(0);

        s->top = s->base + s->stackSize;
        s->stackSize = s->stackSize + STACKINCREMENT;
    }

    *(s->top) = e;      // 存放数据
    s->top++;
}

Pop(sqStack *s, ElemType *e)
{
    if( s->top == s->base )
        return;

    *e = *--(s->top);   // 将栈顶元素弹出并修改栈顶指针
}

int StackLen(sqStack s)
{
    return (s.top - s.base);
}

int main()
{
    sqStack s;
    char c, e;

    InitStack( &s );

    printf("请输入中缀表达式，以#作为结束标志: ");
    scanf("%c", &c);

    while( c != '#')
    {
        while( c>='0' && c<='9' )
        {
            printf("%c", c);
            scanf("%c", &c);
            if( c<'0' || c>'9' )
            {
                printf(" ");
            }
        }
        if( ')' == c )
        {
            Pop(&s, &e);
            while( '(' != e )
            {
                printf("%c", e);
                Pop(&s, &e);
            }
        }
        else if( '+'==c || '-'==c )
        {
            if( !StackLen(s) )
            {
                Push(&s, c);
            }
            else
            {
                do
                {
                    Pop(&s, &e);
                    if( '(' == e)
                    {
                        Push(&s, e);
                    }
                    else
                    {
                        printf("%c", e);
                    }
                }while( StackLen(s) && '('!=e );
                Push(&s, c);
            }
        }
        else if( '*'==c || '/'==c || '('==c )
        {
            Push(&s, c);
        }
        else if( '#'==c )
        {
            break;
        }
        else
        {
            printf("\n出错，输入格式错误！\n");
            return -1;
        }
        scanf("%c", &c);
    }

    while( StackLen(s) )
    {
        Pop(&s, &e);
        printf("%c", e);
    }

    return 0;
}

// 5 - (6 + 7) * 8 + 9 / 4
// 5 - 13 * 8 + 9 / 4
// 5 6 7 + 8 * - 9 4 / +