232. 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).

class MyQueue {
// Push element x to the back of queue.
Stack<Integer> input = new Stack();
Stack<Integer> output = new Stack();
public void push(int x) {
input.push(x);
}

// Removes the element from in front of queue.
public void pop() {
peek();
output.pop();

}

// Get the front element.
public int peek() {
if (output.empty())
while (!input.empty())
output.push(input.pop());
return output.peek();

}

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

总结：关键在于peek函数的应用使得pop前output中始终有值。