[CareerCup] 3.1 Implement Three Stacks using Array 使用数组来实现三个栈

class ThreeStacks {
public:
    ThreeStacks(int size) : _stackSize(size) {
        _buffer.resize(size * 3, 0);
        _stackCur.resize(3, -1);
    }
    
    void push(int stackIdx, int val) {
        if (_stackCur[stackIdx] + 1 >= _stackSize) {
            cout << "Stack " << stackIdx << " is full!" << endl;
        }
        ++_stackCur[stackIdx];
        _buffer[stackIdx * _stackSize + _stackCur[stackIdx]] = val;
    }
    
    void pop(int stackIdx) {
        if (empty(stackIdx)) {
            cout << "Stack " << stackIdx << " is empty!" << endl;
        }
        _buffer[stackIdx * _stackSize + _stackCur[stackIdx]] = 0;
        --_stackCur[stackIdx];
    }
    
    int top(int stackIdx) {
        if (empty(stackIdx)) {
            cout << "Stack " << stackIdx << " is empty!" << endl;
        }
        return _buffer[stackIdx * _stackSize + _stackCur[stackIdx]];
    }
    
    bool empty(int stackIdx) {
        return _stackCur[stackIdx] == -1;
    }
    
private:
    int _stackSize;
    vector<int> _buffer;
    vector<int> _stackCur;
};