一.解法

https://leetcode-cn.com/problems/implement-queue-using-stacks/
要点：辅助栈
Python，C++，Java都用了双栈法，a栈负责进，b栈负责出，需要取首位或者pop首位时，b如果非空直接取b首位，否则将a中元素逆着全部加进b栈。

二.Python实现

class MyQueue:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.instack = []
        self.outstack = []

    def push(self, x):
        """
        Push element x to the back of queue.
        :type x: int
        :rtype: None
        """
        self.instack.append(x)

    def pop(self):
        """
        Removes the element from in front of queue and returns that element.
        :rtype: int
        """
        if len(self.outstack) == 0:
            while self.instack:
                self.outstack.append(self.instack.pop())
        return self.outstack.pop()

    def peek(self):
        """
        Get the front element.
        :rtype: int
        """
        if len(self.outstack) == 0:
            while self.instack:
                self.outstack.append(self.instack.pop())
        return self.outstack[-1]
    def empty(self):
        """
        Returns whether the queue is empty.
        :rtype: bool
        """
        return len(self.instack) == 0 and len(self.outstack) == 0


# Your MyQueue object will be instantiated and called as such:
# obj = MyQueue()
# obj.push(x)
# param_2 = obj.pop()
# param_3 = obj.peek()
# param_4 = obj.empty()

三.C++实现

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

四.java实现

class MyQueue {
    private Stack<Integer> a;// 输入栈
    private Stack<Integer> b;// 输出栈

    /** Initialize your data structure here. */
    public MyQueue() {
        a = new Stack<>();
        b = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
      
        a.push(x);
    
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
         if(b.isEmpty()){
            while(!a.isEmpty()){
                b.push(a.pop());
            }
        }
        return b.pop();

    }
    
    /** Get the front element. */
    public int peek() {
        if(b.isEmpty()){
            while(!a.isEmpty()){
                b.push(a.pop());
            }
        }
        return b.peek();

    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return a.isEmpty() && b.isEmpty();

    }
}

/**
 * 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();
 * boolean param_4 = obj.empty();
 */