如何只用一个数组来实现三个栈

题目：描述如何只用一个数组来实现三个栈





int stackSize=100;
int [] buffer=new int [stackSize*3];
int []stackPointer={-1,-1,-1};//用于追踪栈顶元素的指针
void push(int stackNum,int value) throws Exception
{
/*检查有无空闲空间*/
if(stackPointer[stackNum]+1>=stackSize)//最后一个元素
{
throw new Exception("out of space.");
}
//栈指针自增，然后更新栈顶元素的值
stackPointer[stackNum]++;
buffer[absTopOfStack(stackNum)]=value;
}
int pop(int stackNum) throws Exception
{
if(stackPointer[stackNum]==-1)
{
throw new Exception("Trying to pop an empty stack.");
}
int value=buffer[absTopOfStack(stackNum)];//获取栈顶元素
buffer[absTopOfStack(stackNum)]=0;//清零指定索引元素的值
stackPointer[stackNum]--;//指针自减
return value;
}
int peek(int stackNum)
{
int index=absTopOfStack(stackNum);
return buffer[index];
}
boolean isEmpty(int stackNum)
{
return stackPointer[stackNum]==-1;
}
//返回栈“stackNum”栈顶元素的索引，绝对量

    int absTopOfStack(int stackNum)

    {

    return stackNum*stackSize+stackPointer[stackNum];

    }