使用栈实现队列的下列操作:
- push(x) -- 将一个元素放入队列的尾部。
- pop() -- 从队列首部移除元素。
- peek() -- 返回队列首部的元素。
- empty() -- 返回队列是否为空。
class MyQueue {
Stack<Integer> s1 = null;
Stack<Integer> s2 = null;
/** Initialize your data structure here. */
public MyQueue() {
s1 = new Stack<>();
s2 = new Stack<>()
}
/** Push element x to the back of queue. */
public void push(int x) {
s1.push(x);
}
/** Removes the element from in front of queue and returns that element. */
public int pop() {
if(s2.isEmpty()){
while(!s1.isEmpty()){
s2.push(s1.pop());
}
}
return s2.pop();
}
/** Get the front element. */
public int peek() {
if(s2.isEmpty()){
while(!s1.isEmpty()){
s2.push(s1.pop());
}
}
return s2.peek();
}
/** Returns whether the queue is empty. */
public boolean empty() {
return s1.isEmpty() && s2.isEmpty();
}
}