[Leetcode] 232. Implement Queue using Stacks

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.

Notes:

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

题目大意：使用栈（stack）实现队列（queue）

思路：使用两个栈，入队的元素均保存在栈s1中，出队均从栈s2中弹出。当然，当栈s2为空时，需要将栈s1里的全部元素保存进栈s2中。

class MyQueue {
public:
    stack<int> s1,s2;
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        s1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int x = peek();
        s2.pop();
        return x;
    }
    
    /** Get the front element. */
    int peek() {
        if(s2.empty())
        {
            while(s1.size()>0)
            {
                s2.push(s1.top());
                s1.pop();
            }
        }
        int x = s2.top();
        return x;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty()&&s2.empty();
    }
};

/**
 * 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();
 */