# 232 Implement Queue using Stack

Concept:

只能用stack，但要做到FIFO，想法是用兩個stack，

存的時候就一樣push到stack1裡面，pop和peek的時候就從stack1全部丟到stack2，

從stack2 pop或peek

empty就直接看stack裡面有沒有東西就好。

Code:

public class MyQueue {
    Deque<Integer> stack1;
    Deque<Integer> stack2;
    /** Initialize your data structure here. */
    public MyQueue() {
        stack1 = new ArrayDeque<>();
        stack2 = new ArrayDeque<>();
    }

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

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        while(!stack1.isEmpty()){
            stack2.push(stack1.pop());
        }
        int i = stack2.pop();
        while(!stack2.isEmpty()){
            stack1.push(stack2.pop());
        }
        return i;
    }

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

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack1.isEmpty();
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */