Write routines to implement two stacks using only one array. Your stack routines should not declare an overflow unless every slot in the array is used.

Format of functions:

Stack CreateStack( int MaxElements );
int IsEmpty( Stack S, int Stacknum );
int IsFull( Stack S );
int Push( ElementType X, Stack S, int Stacknum );
ElementType Top_Pop( Stack S, int Stacknum );

where int Stacknum is the index of a stack which is either 1 or 2; int MaxElements is the size of the stack array; and Stack is defined as the following:

typedef struct StackRecord *Stack;
struct StackRecord  {
    int Capacity;       /* maximum size of the stack array */
    int Top1;           /* top pointer for Stack 1 */
    int Top2;           /* top pointer for Stack 2 */
    ElementType *Array; /* space for the two stacks */
}

Note: Push is supposed to return 1 if the operation can be done successfully, or 0 if fails. If the stack is empty, Top_Pop must return ERROR which is defined by the judge program.

Sample program of judge:

#include <stdio.h>
#include <stdlib.h>
#define ERROR 1e8
typedef int ElementType;
typedef enum { push, pop, end } Operation;

typedef struct StackRecord *Stack;
struct StackRecord  {
    int Capacity;       /* maximum size of the stack array */
    int Top1;           /* top pointer for Stack 1 */
    int Top2;           /* top pointer for Stack 2 */
    ElementType *Array; /* space for the two stacks */
};

Stack CreateStack( int MaxElements );
int IsEmpty( Stack S, int Stacknum );
int IsFull( Stack S );
int Push( ElementType X, Stack S, int Stacknum );
ElementType Top_Pop( Stack S, int Stacknum );

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

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

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

/* Your function will be put here */

Sample Input:

5
Push 1 1
Pop 2
Push 2 11
Push 1 2
Push 2 12
Pop 1
Push 2 13
Push 2 14
Push 1 3
Pop 2
End

Sample Output:

Stack 2 is Empty!
Stack 1 is Full!
Pop from Stack 1: 1
Pop from Stack 2: 13 12 11


代码：

Stack CreateStack( int MaxElements )
{
    Stack p = (Stack)malloc(sizeof(struct StackRecord));
    p -> Array = (int *)malloc(sizeof(MaxElements));
    p -> Capacity = MaxElements;
    p -> Top1 = -1;
    p -> Top2 = MaxElements;
    return p;
}
int IsEmpty( Stack S, int Stacknum )
{
    if(Stacknum == 1)
    {
        if(S -> Top1 == -1)return 1;
        return 0;
    }
    else
    {
        if(S -> Top2 == S -> Capacity)return 1;
        return 0;
    }
}
int IsFull( Stack S )
{
    if(S -> Top2 - S -> Top1 == 1)return 1;
    return 0;
}
int Push( ElementType X, Stack S, int Stacknum )
{
    if(IsFull(S))return 0;
    if(Stacknum == 1)S -> Array[++ (S -> Top1)] = X;
    else S -> Array[-- (S -> Top2)] = X;
    return 1;
}
ElementType Top_Pop( Stack S, int Stacknum )
{
    if(IsEmpty(S,Stacknum))return ERROR;
    if(Stacknum == 1)return S -> Array[(S -> Top1) --];
    else return S -> Array[(S -> Top2) ++];
}