Java [Leetcode 232]Implement Queue using Stacks

题目描述：

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).

解题思路：

使用两个stack，一个用于插入数据，另一个用于输出数据。

代码如下：

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