232. Implement Queue using Stacks

1.描述

Implement the following operations of a queue using stacks.

push(x) -- Push element x to the back of queue.
pop() -- Removes the element from in front of queue.
peek() -- Get the front element.
empty() -- Return whether the queue is empty.

2.分析

3.代码

class MyQueue {
public:
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        while (!s2.empty()) {
            s1.push(s2.top());
            s2.pop();
        }
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if (empty()) exit(-1);
        while (!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
        }
        int ans = s2.top();
        s2.pop();
        return ans;
    }
    
    /** Get the front element. */
    int peek() {
        if (empty()) exit(-1);
        while (!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
        }
        return s2.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty() && s2.empty();
    }
private:
    stack s1, s2;
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * bool param_4 = obj.empty();
 */