[LeetCode 解题报告]225. Implement Stack using Queues

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        que2.push(x);
        while (que2.size() > 1) {
            que1.push(que2.front());
            que2.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int tmp = top();
        que2.pop();
        return tmp;
    }
    
    /** Get the top element. */
    int top() {
        if (que2.empty()) {
            for (int i = 0; i < que1.size()-1; i ++) {
                que1.push(que1.front());
                que1.pop();
            }
            que2.push(que1.front());
            que1.pop();
        }
        return que2.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return que1.empty() && que2.empty();
    }

private:
    queue<int> que1, que2;
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */