How to implement 3 stacks with one array?

Question

Sometimes, I come across the following interview question: How to implement 3 stacks with one array ? Of course, any static allocation is not a solution.

Answer 1

enum stackId{LEFT, MID, RIGHT };

class threeStacks {

int* arr;

int leftSize;
int rightSize;
int midSize;
int mid;
int maxSize;
public:
    threeStacks(int n):leftSize(0), rightSize(0), midSize(0), mid(n/2), maxSize(n)
    {
        arr = new int[n];
    }

    void push(stackId sid, int val){
        switch(sid){
            case LEFT:
                pushLeft(val);
            break;

            case MID:
                pushMid(val);
            break;

            case RIGHT:
                pushRight(val);
        }
    }

    int pop(stackId sid){
        switch(sid){
            case LEFT:
                return popLeft();
            case MID:
                return popMid();
            case RIGHT:
                return popRight();
        }
    }


    int top(stackId sid){
        switch(sid){
            case LEFT:
                return topLeft();
            case MID:
                return topMid();
            case RIGHT:
                return topRight();
        }
    }

    void pushMid(int val){
        if(midSize+leftSize+rightSize+1 > maxSize){
            cout << "Overflow!!"< 1)
                    freeSpace /= 2;
                for(int i=0; i< midSize; i++){
                    arr[(mid - midSize/2) - freeSpace + i] = arr[(mid - midSize/2) + i];
                }
                mid = mid-freeSpace;
            break;
            case RIGHT:
                freeSpace = maxSize - rightSize - (mid + midSize/2) - 1;
                if(freeSpace < 1)
                    return false;               
                if(freeSpace > 1)
                    freeSpace /= 2;
                for(int i=0; i< midSize; i++){
                    arr[(mid + midSize/2) + freeSpace - i] = arr[(mid + midSize/2) - i];
                }
                mid = mid+freeSpace;
            break;              
            default:
                return false;
        }
    }

    void pushLeft(int val){
        if(midSize+leftSize+rightSize+1 > maxSize){
            cout << "Overflow!!"< maxSize){
            cout << "Overflow!!"<

};