QueueUsingStack.java

package stackqueues.queueusingstack;

import java.util.Stack;

/**
 * Implement a queue using stack. The idea is to use two stacks.
 * Push everything on stack1.
 * Pop everything from stack1 into push on stack2 if stack2 is not empty.
 * Queue A B C
 * C -> A -> A B C
 * B pop1 B pop
 * A push2 C
 * Stack 1 Stack 2
 * User: rpanjrath
 * Date: 9/19/13
 * Time: 1:56 PM
 */
public class QueueUsingStack {

 private Stack<String> stack1 = new Stack<>();
 private Stack<String> stack2 = new Stack<>();

 public void add(String temp) {
 stack1.push(temp);
 }

 public String remove() {
 if (stack2.isEmpty()) { // V Imp else we can concurrently modify the stacks
 while (!stack1.isEmpty()) {
 stack2.push(stack1.pop());
 }
 }
 return stack2.pop();
 }

 public static void main(String[] args) {
 QueueUsingStack queueUsingStack = new QueueUsingStack();
 queueUsingStack.add("A");
 queueUsingStack.add("B");
 queueUsingStack.add("C");
 System.out.print(queueUsingStack.remove());
 System.out.print(queueUsingStack.remove());
 queueUsingStack.add("D");
 System.out.print(queueUsingStack.remove());
 System.out.print(queueUsingStack.remove());
 }
}