LeetCode_232. Implement Queue using Stacks

232. Implement Queue using Stacks

Easy

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).

package leetcode.easy;

public class ImplementQueueUsingStacks {
	@org.junit.Test
	public void test1() {
		MyQueue1 queue = new MyQueue1();

		queue.push(1);
		queue.push(2);
		System.out.println(queue.peek()); // returns 1
		System.out.println(queue.pop()); // returns 1
		System.out.println(queue.empty()); // returns false
	}

	@org.junit.Test
	public void test2() {
		MyQueue2 queue = new MyQueue2();

		queue.push(1);
		queue.push(2);
		System.out.println(queue.peek()); // returns 1
		System.out.println(queue.pop()); // returns 1
		System.out.println(queue.empty()); // returns false
	}
}

class MyQueue1 {
	private java.util.Stack<Integer> s1 = new java.util.Stack<>();
	private java.util.Stack<Integer> s2 = new java.util.Stack<>();
	private int front;

	/** Initialize your data structure here. */
	public MyQueue1() {

	}

	/** Push element x to the back of queue. */
	public void push(int x) {
		if (s1.empty()) {
			front = x;
		}
		while (!s1.isEmpty()) {
			s2.push(s1.pop());
		}
		s2.push(x);
		while (!s2.isEmpty()) {
			s1.push(s2.pop());
		}
	}

	// Removes the element from the front of queue.
	public int pop() {
		int temp = s1.pop();
		if (!s1.empty()) {
			front = s1.peek();
		}
		return temp;
	}

	// Get the front element.
	public int peek() {
		return front;
	}

	// Return whether the queue is empty.
	public boolean empty() {
		return s1.isEmpty();
	}
}

class MyQueue2 {
	private java.util.Stack<Integer> s1 = new java.util.Stack<>();
	private java.util.Stack<Integer> s2 = new java.util.Stack<>();
	private int front;

	/** Initialize your data structure here. */
	public MyQueue2() {

	}

	// Push element x to the back of queue.
	public void push(int x) {
		if (s1.empty()) {
			front = x;
		}
		s1.push(x);
	}

	// Removes the element from in front of queue.
	public int pop() {
		if (s2.isEmpty()) {
			while (!s1.isEmpty()) {
				s2.push(s1.pop());
			}
		}
		return s2.pop();
	}

	// Get the front element.
	public int peek() {
		if (!s2.isEmpty()) {
			return s2.peek();
		}
		return front;
	}

	// Return whether the queue is empty.
	public boolean empty() {
		return s1.isEmpty() && s2.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();
 */