中缀表达式 转后缀表达式

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

typedef struct Stack
{
    char *base, *top;
    int size;
}Sta;

void init(Sta *s)
{
    s->base =(Sta *) malloc (20*sizeof(char));
    s->top = s->base;
    s->size = 20;
}

void push(Sta *s, char e)
{
    if (s->top - s->base >= s->size)
    {
        s->base = (Sta *)realloc (s, (s->size+10)*sizeof(char));
        s->size += 10;
    }
    *(s->top) = e;
    s->top ++;
}

void pop(Sta *s, char *e)
{
    *e = *--(s->top);
}

int len(Sta s)
{
    return (s.top - s.base);
}

int main()
{
    Sta s;
    char e, d, c;
    init (&s);
    scanf("%c", &c);

    while (c!= '#')
    {
        while (isdigit(c))
        {
            printf("%c", c);
            scanf("%c", &c);
            if (!isdigit(c))
                printf(" ");
        }
        if (')' == c)
        {
            pop(&s, &e);
            while ('(' != e)
            {
                printf("%c ", e);
                pop(&s, &e);
            }
        }
        else if ('-'==c || '+'==c)
        {
            if (!len(s))
            {
                push(&s, c);
            }
            else
            {
                do
                {
                    pop(&s, &e);
                    if ('('==e)
                    {
                        push(&s, e);
                    }
                    else
                    {
                        printf("%c ", e);
                    }
                }while(len(s) && '(' != e);
                push(&s, c);
            }
        }
        else if ('*'==c || '/'==c || '('==c)
        {
            push(&s, c);
        }
        else if ('#'== c)
        {
            break;
        }
        scanf("%c", &c);
    }
    while (len(s))
    {
        pop(&s, &e);
        printf("%c ", e);
    }
}