_232.java

package com.fishercoder.solutions;

import java.util.Stack;
/**
 * 232. Implement Queue using Stacks
 *
 * 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.

 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).
 */
public class _232 {

    public static class Solution1 {
        class MyQueue {

            Stack<Integer> input = new Stack();
            Stack<Integer> output = new Stack();

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

            // Removes the element from in front of queue.
            public int pop() {
                peek();
                return output.pop();
            }

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

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