LeetCode-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).

这种题可以自己先跑几个样例就会发现是有规律可循的，主要思路是两个stack，一个stack1用于push，另外一个stack2用于pop，关键就在从stack1倒到stack2的过程，还是直接上代码吧，一目了然：

class MyQueue {

LinkedList<Integer> stack1 = new LinkedList<Integer>();
LinkedList<Integer> stack2 = new LinkedList<Integer>();

// Push element x to the back of queue.
public void push(int x) {
stack1.push(x);
}

// Removes the element from in front of queue.
public void pop() {
if (stack2.isEmpty()) {
while (!stack1.isEmpty()) {
stack2.push(stack1.pop());
}
}
stack2.pop();
}

// Get the front element.
public int peek() {
if (stack2.isEmpty()) {
while (!stack1.isEmpty()) {
stack2.push(stack1.pop());
}
}
return stack2.peek();
}

// Return whether the queue is empty.
public boolean empty() {
return stack1.isEmpty() && stack2.isEmpty();
}
}