225. Implement Stack using Queues

1.描述

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.
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).

2.分析

3.代码

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q1.push(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        if (empty()) exit(-1);
        int ans;
        if (!q1.empty()) {
            ans = q1.back();
            while (q1.size() > 1) {
                q2.push(q1.front());
                q1.pop();
            }
            q1.pop();            
        } else {
            ans = q2.back();
            while (q2.size() > 1) {
                q1.push(q2.front());
                q2.pop();
            }
            q2.pop();
        }
        return ans;
    }
    
    /** Get the top element. */
    int top() {
        if (empty()) exit(-1);
        if (q1.size() >= 1)
            return q1.back();
        else if (q2.size() >= 1)
            return q2.back();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q1.empty() && q2.empty();
    }
private:
    queue q1, q2;
};

/**
 * 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();
 */