pta Two Stacks In One Array(简单版双向数组）

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`这里写代码片`

Sample Output:

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

思路：
双向数组的思想，但这里的结构其实比双向数组简单，因为两个栈是分开的，栈1的指针不会把栈2的东西pop了。主要考点是理清楚双向数组为Empty和Full的判断条件。
这里我用的是增加一个多余格的方法，这样一来空的条件是在初始位置，Full的条件的Top1 == Top2。

代码：

ElementType Top_Pop( Stack S, int Stacknum )
{
    if(IsEmpty(S,Stacknum))return ERROR;

    int re;
    if(Stacknum == 1)
    {
        S->Top1++;
        if(S->Top1>S->Capacity)S->Top1 = 0;
        re = S->Array[S->Top1];
    }
    else
    {
        S->Top2--;
        if(S->Top2<0)S->Top2 = S->Capacity;
        re = S->Array[S->Top2];
    }
    return re;
}

int Push( ElementType X, Stack S, int Stacknum )
{
    if(IsFull(S))
    {
        return 0;
    }

    if(Stacknum == 1)
    {
        S->Array[S->Top1--] = X;
        if(S->Top1 < 0)S->Top1 = S->Capacity;
    }
    else
    {
        S->Array[S->Top2++] = X;
        if(S->Top2 > S->Capacity)S->Top2 = 0;
    }
    return 1;
}

int IsFull( Stack S )
{
    if(S->Top1 == S->Top2)return 1;
    return 0;
}

int IsEmpty( Stack S, int Stacknum )
{
    if(Stacknum == 1)
    {
        if(S->Top1 == S->Capacity-1)return 1;
    }
    else if(S->Top2 == S->Capacity)return 1;
    return 0;
}

Stack CreateStack( int MaxElements )
{
    Stack p = (Stack)malloc(sizeof(struct StackRecord));
    p->Capacity = MaxElements;
    p->Top1 = MaxElements-1;
    p->Top2 = MaxElements;
    p->Array = (int *)malloc((MaxElements+1)*sizeof(int));
    return p;
}