Leetcode: 232. Implement Queue using Stacks(JAVA)

【问题描述】

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.
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).

【思路】
用两个stack实现

class MyQueue {
Stack<Integer> s1 = new Stack<Integer>();
Stack<Integer> s2 = new Stack<Integer>();

public void push(int x) {
s1.push(x);
}

// Removes the element from in front of queue.
public void pop() {
if (!s2.isEmpty()) {
s2.pop();
} else {
while (!s1.isEmpty()) {
s2.push(s1.pop());
}
s2.pop();
}
}

// Get the front element.
public int peek() {
if (!s2.isEmpty()) {
return s2.peek();
}else {
while(!s1.isEmpty()){
s2.push(s1.pop());
}
return s2.peek();
}
}

// Return whether the queue is empty.
public boolean empty() {
return s1.empty() && s2.empty();
}
}