crack the code interview 3.1

//Describe how you could use a single array to implement three stacks.
#include <iostream>
using namespace std;

int stackSize = 900;
int indexUsed = 0;//已经存放的value的个数
int stackTop[3] = {-1, -1, -1};
StackNode * buffer = new StackNode[stackSize];

void push(int stack, int value)
{
    if (indexUsed > stackSize)
    {
        cout<<"Stack is full"<<endl;
        return;
    }
    int lastIndex = stackTop[stack];
    stackTop[stack] = indexUsed;
    indexUsed ++;
    buffer[stackTop[stack]] = new StackNode(lastIndex, value);
}

int pop(int stack)
{
    int lastIndex = stackTop[stack];
    int value = buffer[stackTop[stack]].value;
    stackTop[stack] = buffer[stackTop[stack]].previous;
    indexUsed --;
    return value;
}

int top(int stack)
{
    return buffer[stackTop[stack]].value;
}

bool isEmpty(int stack)
{
    return stackTop[stack] != -1;
}

struct StackNode{
    int previous;
    int value;
    StackNode(int _pre, int _val)
    {
        previous = _pre;
        value = _val;
    }
};

这个代码只是实现了简单的插入和回收工作，但是当删除以后的空余空间无法再次使用，这是这个代码的缺点。