两栈共享存储空间

数组有两个端点，两个栈有两个栈底，让一个栈的栈底为数组的始端，即下标为0处，另一个栈为栈的末端，即下标为数组长度

n-1处。这样，如果两个栈增加元素，就是两端点向中间延伸。当top1 + 1 == top2 的时候为栈满。

示例代码：（改编自《大话数据结构》）

 C++ Code 

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#include <iostream>
using namespace std;

#define  MAXSIZE 20

typedef int ElemType;
typedef struct
{
    ElemType data[MAXSIZE];
    int top1; //栈1栈顶指针
    int top2; //栈2栈顶指针
} SqStack;

/* 构造一个空栈*/
bool InitStack(SqStack *Sq)
{
    cout << "Init Stack ..." << endl;
    Sq->top1 = -1; //表示空栈
    Sq->top2 = MAXSIZE;

    return true;
}
/* 置为空栈 */
bool ClearStack(SqStack *Sq)
{
    cout << "Clear Stack ..." << endl;
    Sq->top1 = -1;
    Sq->top2 = MAXSIZE;
    return true;
}

bool StackEmpty(SqStack Sq)
{
    if (Sq.top1 == -1 && Sq.top2 == MAXSIZE)
        return true;
    else
        return false;
}

int StackLength(SqStack Sq)
{
    cout << "Stack Length : ";
    return Sq.top1 + 1 + MAXSIZE - Sq.top2;
}
/* 返回栈顶元素 */
bool GetTop(SqStack Sq, ElemType *ptr, int StackNum)
{
    if (StackNum == 1)
    {
        if (Sq.top1 != -1)
        {
            *ptr = Sq.data[Sq.top1];
            cout << "Get Top1 Item " << *ptr << endl;
            return true;
        }
        return false;
    }

    else if (StackNum == 2)
    {
        if (Sq.top2 != MAXSIZE)
        {
            *ptr = Sq.data[Sq.top2];
            cout << "Get Top2 Item " << *ptr << endl;
            return true;
        }
        return false;
    }
    else
    {
        cout << "Stack Num must be 1 or 2!" << endl;
        return false;
    }
}

/* 压栈 */
bool Push(SqStack *Sq, ElemType Elem, int StackNum)
{
    if (StackNum == 1)
    {
        cout << "Push Item to Stack1 : " << Elem << endl;
        if (Sq->top1 + 1 == Sq->top2)   //栈满
            return false;
        Sq->data[++Sq->top1] = Elem;
        return true;
    }
    else if (StackNum == 2)
    {
        cout << "Push Item to Stack2 : " << Elem << endl;
        if (Sq->top1 + 1 == Sq->top2)
            return false;
        Sq->data[--Sq->top2] = Elem;
        return true;
    }
    else
    {
        cout << "Stack Num must be 1 or 2!" << endl;
        return false;
    }
}
/* 出栈 */
bool Pop(SqStack *Sq, ElemType *ptr, int StackNum)
{
    if (StackNum == 1)
    {
        if (Sq->top1 == -1)
            return false;

        *ptr = Sq->data[Sq->top1--];
        cout << "Pop Item from Stack1 :  " << *ptr << endl;
        return true;
    }
    else if (StackNum == 2)
    {
        if (Sq->top2 == MAXSIZE)
            return false;
        *ptr = Sq->data[Sq->top2++];
        cout << "Pop Item from Stack2 :  " << *ptr << endl;
        return true;
    }
    else
    {
        cout << "Stack Num must be 1 or 2!" << endl;
        return false;
    }

}

bool StackTraverse(SqStack Sq)
{
    cout << "Traverse Stack ..." << endl;
    if (StackEmpty(Sq))
        return false;
    cout << "Stack1 : ";
    for (int i = 0; i <= Sq.top1; i++)
        cout << Sq.data[i] << ' ';
    cout << endl;
    cout << "Stack2 : ";
    for (int i = MAXSIZE - 1; i >= Sq.top2; i--)
        cout << Sq.data[i] << ' ';
    cout << endl;

    return true;
}

int main(void)
{
    SqStack Sq;
    InitStack(&Sq);
    for (int i = 0; i < 5; i++)
        Push(&Sq, i, 1);
    for (int i = 5; i < 10; i++)
        Push(&Sq, i, 2);
    StackTraverse(Sq);
    int result;
    Pop(&Sq, &result, 1);
    Pop(&Sq, &result, 2);
    StackTraverse(Sq);
    GetTop(Sq, &result, 1);
    GetTop(Sq, &result, 2);
    if (!StackEmpty(Sq))
        cout << StackLength(Sq) << endl;

    ClearStack(&Sq);

    return 0;
}

输出为：

事实上，使用这样的数据结构，通常都是当两个栈的空间需求有想法关系时，也就是当一个栈增长时另一个栈在缩短的情况。

还需要注意的一点是必须是同种数据类型的栈，否则不但不能更好地解决问题，反而会使问题更加复杂。