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# 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.
Example:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // returns 1
queue.pop(); // returns 1
queue.empty(); // returns false
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).

## 代码

public class Solution {
private Stack<Integer> stack1;
private Stack<Integer> stack2;
public Solution() {
stack1 = new Stack<Integer>();
stack2 = new Stack<Integer>();
}
public void push(int element) {
stack1.push(element);
}
public int pop() {
if (stack2.empty()) {
stack1ToStack2(stack1, stack2);
}
return stack2.pop();
}
public int top() {
if (stack2.empty()) {
stack1ToStack2(stack1, stack2);
}
return stack2.peek();
}
private void stack1ToStack2() {
while (!stack1.empty()) {
stack2.push(stack1.pop());
}
}
}