问题：

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 emptyoperations 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).

解决：

【注】关于测试参数的解释：

["MyQueue","push","push","peek","pop","pop","empty"] --- 表示要进行的操作

[[],[1],[2],[],[],[],[]] --- 表示对应传入的参数

① 使用两个栈s1，s2，进栈的时候不做任何处理，出栈的时候把栈逆序放在另外一个栈，出另外一个栈。
之前写经常出现空栈异常或者超时，就是没有写好s1，s2之间的转换。

class MyQueue { // 87ms
    Stack<Integer> s1 = new Stack<>();
    Stack<Integer> s2 = new Stack<>();
    /** Initialize your data structure here. */
    public MyQueue() {}
    /** Push element x to the back of queue. */
    public void push(int x) {
        while(! s2.isEmpty()) s1.push(s2.pop());
        s1.push(x);
    }
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        while(! s1.isEmpty()) s2.push(s1.pop());
        return s2.pop();
    }
    /** Get the front element. */
    public int peek() {
        while(! s1.isEmpty()) s2.push(s1.pop());
        return s2.peek();
    }
    /** Returns whether the queue is empty. */
    public boolean empty() {
        while(! s2.isEmpty()) s1.push(s2.pop());
        return s1.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();
 */

② 使用两个栈，在进栈的时候，把栈逆序放在另外一个栈，出的时候直接出另外一个栈就可以了。

class MyQueue { //93ms
    Stack<Integer> s1 = new Stack<>();
    Stack<Integer> s2 = new Stack<>();
    /** Initialize your data structure here. */
    public MyQueue() {}
    /** Push element x to the back of queue. */
    public void push(int x) {
        while(! s1.isEmpty()) s2.push(s1.pop());
        s2.push(x);
        while(! s2.isEmpty()) s1.push(s2.pop());
    }
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return s1.pop();
    }
    /** Get the front element. */
    public int peek() {
        return s1.peek();
    }
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty();
    }
}

③ 除了使用两个栈之外，额外使用一个指针指向头部。

class MyQueue { //113ms
    Stack<Integer> s1 = new Stack<>();
    Stack<Integer> s2 = new Stack<>();
    int head;
    /** Initialize your data structure here. */
    public MyQueue() {}
    /** Push element x to the back of queue. */
    public void push(int x) {
        if(s1.isEmpty()) head = x;
        s1.push(x);
    }
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        while(! s1.isEmpty()) s2.push(s1.pop());
        int top = -1;
        if(! s2.isEmpty()){
            top = s2.pop();
            if(! s2.isEmpty()) head = s2.peek();
        }
        while(! s2.isEmpty()) s1.push(s2.pop());
        return top;
    }
    /** Get the front element. */
    public int peek() {
        return head;
    }
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return s1.isEmpty();
    }
}