LeetCode_225. Implement Stack using Queues

225. Implement Stack using Queues

Easy

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

package leetcode.easy;

public class ImplementStackUsingQueues {
	@org.junit.Test
	public void test1() {
		MyStack1 stack = new MyStack1();

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

	@org.junit.Test
	public void test2() {
		MyStack2 stack = new MyStack2();

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

	@org.junit.Test
	public void test3() {
		MyStack3 stack = new MyStack3();

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

class MyStack1 {
	private java.util.Queue<Integer> q1 = new java.util.LinkedList<>();
	private java.util.Queue<Integer> q2 = new java.util.LinkedList<>();
	private int top;

	/** Initialize your data structure here. */
	public MyStack1() {

	}

	// Push element x onto stack.
	public void push(int x) {
		q1.add(x);
		top = x;
	}

	// Removes the element on top of the stack.
	public int pop() {
		while (q1.size() > 1) {
			top = q1.remove();
			q2.add(top);
		}
		int tempPop = q1.remove();
		java.util.Queue<Integer> temp = q1;
		q1 = q2;
		q2 = temp;
		return tempPop;
	}

	/** Get the top element. */
	public int top() {
		return top;
	}

	/** Returns whether the stack is empty. */
	public boolean empty() {
		return q1.isEmpty();
	}
}

class MyStack2 {
	private java.util.Queue<Integer> q1 = new java.util.LinkedList<>();
	private java.util.Queue<Integer> q2 = new java.util.LinkedList<>();
	private int top;

	/** Initialize your data structure here. */
	public MyStack2() {

	}

	/** Push element x onto stack. */
	public void push(int x) {
		q2.add(x);
		top = x;
		while (!q1.isEmpty()) {
			q2.add(q1.remove());
		}
		java.util.Queue<Integer> temp = q1;
		q1 = q2;
		q2 = temp;
	}

	// Removes the element on top of the stack.
	public int pop() {
		int temp = q1.remove();
		if (!q1.isEmpty()) {
			top = q1.peek();
		}
		return temp;
	}

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

	// Return whether the stack is empty.
	public boolean empty() {
		return q1.isEmpty();
	}
}

class MyStack3 {
	private java.util.Queue<Integer> q1 = new java.util.LinkedList<>();
	private int top;

	/** Initialize your data structure here. */
	public MyStack3() {

	}

	// Push element x onto stack.
	public void push(int x) {
		q1.add(x);
		int sz = q1.size();
		while (sz > 1) {
			q1.add(q1.remove());
			sz--;
		}
		top = x;
	}

	// Removes the element on top of the stack.
	public int pop() {
		int temp = q1.remove();
		if (!q1.isEmpty()) {
			top = q1.peek();
		}
		return temp;
	}

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

	// Return whether the stack is empty.
	public boolean empty() {
		return q1.isEmpty();
	}
}

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