C语言在一个数组中实现两个栈

在一个数组中实现两个栈，并实现入栈，出栈操作

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

#define ERROR 1e8
typedef int ElementType;
typedef enum { push, pop, end } Operation;
typedef enum { false, true } bool;
typedef int Position;
struct SNode {
    ElementType *Data;
    Position Top1, Top2;
    int MaxSize;
};
typedef struct SNode *Stack;

Stack CreateStack( int MaxSize );
bool Push( Stack S, ElementType X, int Tag );
ElementType Pop( Stack S, int Tag );

Operation GetOp();  /* details omitted */
void PrintStack( Stack S, int Tag ); /* details omitted */

int main()
{
    int N, Tag, X;
    Stack S;
    int done = 0;

    scanf("%d", &N);
    S = CreateStack(N);
    while ( !done ) {
        switch( GetOp() ) {
        case push: 
            scanf("%d %d", &Tag, &X);
            if (!Push(S, X, Tag)) printf("Stack %d is Full!\n", Tag);
            break;
        case pop:
            scanf("%d", &Tag);
            X = Pop(S, Tag);
            if ( X==ERROR ) printf("Stack %d is Empty!\n", Tag);
            break;
        case end:
            PrintStack(S, 1);
            PrintStack(S, 2);
            done = 1;
            break;
        }
    }
    return 0;
}

Stack CreateStack( int MaxSize ){
    Stack S = (Stack)malloc(sizeof(struct SNode));
    S->MaxSize = MaxSize;
    S->Data = (ElementType *)malloc(sizeof(ElementType)*MaxSize);
    S->Top2 = MaxSize;
    S->Top1 = -1;
    return S;
}
bool Push( Stack S, ElementType X, int Tag ){
    if(S == NULL)
        return false;
    if((S->Top1 + 1) == S->Top2){
        printf("Stack Full\n");
        return false;
    }
    if(1 == Tag){
        S->Data[++(S->Top1)] = X;
        return true;
    }
    else if (2 == Tag){
        S->Data[(--S->Top2)] = X;
        return true;
    }
    return false;
}
ElementType Pop( Stack S, int Tag ){
    if(S == NULL)
        return ERROR;
    if(1 == Tag){
        if(-1 == S->Top1){
            printf("Stack %d Empty\n", Tag);
            return ERROR;
        }
        return S->Data[(S->Top1)--];    
    }else if(2 == Tag){
        if(S->MaxSize == S->Top2){
            printf("Stack %d Empty\n", Tag);
            return ERROR;
        }
        return S->Data[(S->Top2)++];    
    }
    return ERROR;
}