2个堆栈共用一条内存的问题

//2个堆栈共用一条内存的问题
#include <stdio.h>
#include <stdlib.h>
#define MAXNUM 20
#define elemtype int

typedef struct
{
    elemtype Stack[MAXNUM];
    int  LeftTop;
    int RightTop;
}qstype;

void InitiateQStack(qstype *s)//初始化
{
    s->LeftTop=-1;
    s->RightTop=MAXNUM;   
}

int IsEmpty(qstype s, char WhichStack)
{
    if( WhichStack == 'L' )
    {
        if( s.LeftTop < 0 )
            return 1;
        return 0;
    }
    else if ( WhichStack == 'R' )
    {
        if( s.RightTop >= MAXNUM )
            return 1;
        return 0;
    }
    else
    {
        exit(0);
    }
}
int PushQStack(qstype *s,char WhichStack,elemtype x)//进栈
{
    //if(s->LeftTop>=s->RightTop-1)
    if( s->LeftTop == s->RightTop-1 )
    {
          printf("堆栈已满\n");                           
          return 0;
    }
    if(WhichStack!='L' && WhichStack!='R')
    {
          printf("参数错误\n");
          return 0;                                   
    }
    if(WhichStack == 'L')
        //s->Stack[++s->LeftTop]==x;
        s->Stack[++s->LeftTop] = x;//栈顶不为空
    else
        //s->Stack[--(s->RightTop)]==x;
        s->Stack[--s->RightTop] = x;

    return 1;
}

elemtype PopQStack(qstype *s,char WhichStack)//出栈
{
    if(WhichStack!='L' && WhichStack!='R')
    {
          printf("参数错误\n");
          return 0;                                   
    }
    if(WhichStack=='L')
    {
          if(s->LeftTop<0)
          {
               printf("左栈已空\n");
               return 0;               
          }        
          return s->Stack[(s->LeftTop)--];
    }
    if(WhichStack=='R')
    {
          if(s->RightTop>MAXNUM)
          {
               printf("右栈已空\n");
               return 0;              
          }         
          return s->Stack[(s->RightTop)++];        
    }
}

int main()
{
    char WhichStack;
    qstype *ss = (qstype *)malloc(sizeof(qstype));
    elemtype x,y;

    InitiateQStack(ss);

    printf("请选择插入左表还是右表: 'L'or 'R':\n");
    scanf("%c", &WhichStack);
    while( scanf("%d",&x)!=EOF )
    {
          //scanf("%d",&x);
          PushQStack(ss,WhichStack,x);   
    }
    printf("请选择出栈的表：'L' or 'R':\n");
    scanf("%c",&WhichStack);
    while( !IsEmpty(*ss,WhichStack) )
    {
          y=PopQStack(ss,WhichStack);
          printf("%d\n",y);                              
    }
    getchar();
    getchar();

    return 0;
}

/*
结果运行错误  怎么回事~~~~~~~
*/