232.implement-queue-using-stacks.java

/*
 * @lc app=leetcode id=232 lang=java
 *
 * [232] Implement Queue using Stacks
 *
 * https://leetcode.com/problems/implement-queue-using-stacks/description/
 *
 * algorithms
 * Easy (42.07%)
 * Total Accepted:    140.8K
 * Total Submissions: 332.4K
 * Testcase Example:  '["MyQueue","push","push","peek","pop","empty"]\n[[],[1],[2],[],[],[]]'
 *
 * 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).
 *
 *
 */
class MyQueue {
  private Stack<Integer> in = new Stack<>();
  private Stack<Integer> out = new Stack<>();

  /** Initialize your data structure here. */
  public MyQueue() {
    in = new Stack<>();
    out = new Stack<>();
  }

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

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

  /** Get the front element. */
  public int peek() {
    if (out.isEmpty()) {
      while(!in.isEmpty()) {
        out.push(in.pop());
      }
    }
    return out.peek();
  }

  /** Returns whether the queue is empty. */
  public boolean empty() {
    return in.isEmpty() && out.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();
 */