Q3.1 一个数组实现三个栈

Describe how you could use a single array to implement three stacks.

typedef struct Node{
    int val,preIdex;
    Node(int val = INT_MIN,int preIndex=-2):val(val),preIndex(preIndex){}
};

class Stack3{
public:
    Stack3(int totalSize = 900){
        buf = new Node[totalSize];
        ptop[0] = -1;
        ptop[1] = -1;
        ptop[2] = -1;
        this->totalSize = totalSize;
    }
    
    ~Stack3(){
        delete[] buf;
    }
    
    void Push(int stackNum,int val){
        bur[cur].val = val;
        bur[cur].preIndex = ptop[stackNum];
        ptop[stackNum] = cur++;
        
        //沿着数组一次向后查找，直到找到第一个空的
        while(cur<totalSize){
            if(buf[i].preIndex == -2)
                break;
        }
    }
    
    void Pop(int stackNum){
        
        //每次执行pop操作，检查该元素对应索引是否小于cur，如果是， 将cur更新到该元素索引；否则cur不变
        if(ptop[stackNum]<cur)
            cur = ptop[stackNum];
        //清楚数据     
        buf[ptop[stackNum]].val = INT_MIN;
        ptop[stackNum] = buf[ptop[stackNum]].preIndex;
        buf[ptop[stackNum]].preIndex = -2;
    
    }
    
    int top(int stackNum){
        return buf[ptop[stackNum]].val;
    }
    
    bool empty(int stackNum){
        return ptop[stackNum]== -1;
    }

private:
    Node *buf;
    int ptop[3];
    int totalSize;
    int cur;
};