[232. Implement Queue using Stacks] #leetcode

232_1011.java

class MyQueue {
    private Deque<Integer> deque1;
    private Deque<Integer> deque2;
    private int count;
    
    /** Initialize your data structure here. */
    public MyQueue() {
        deque1 = new LinkedList<Integer>();
        deque2 = new LinkedList<Integer>();
        count = 0;
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        deque1.addLast(x);
        count++;
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        changeDeque();
        count--;
        return deque2.removeLast();
    }
    
    /** Get the front element. */
    public int peek() {
        changeDeque();
        return deque2.peekLast();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return count == 0;
    }
    
    private void changeDeque() {
        if (deque2.size() == 0) {
            int n = deque1.size();
            for (int i = 0; i < n; i++) {
                deque2.addLast(deque1.removeLast());
            }
        }
    }
}

/**
 * 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();
 */