145 lines
3.6 KiB
Java
145 lines
3.6 KiB
Java
import java.util.Stack;
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/**
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* This implements Queue using two Stacks.
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*
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* Big O Runtime:
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* insert(): O(1)
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* remove(): O(1) amortized
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* isEmpty(): O(1)
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*
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* A queue data structure functions the same as a real world queue.
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* The elements that are added first are the first to be removed.
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* New elements are added to the back/rear of the queue.
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*
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* @author sahilb2
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*
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*/
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class QueueWithStack {
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// Stack to keep track of elements inserted into the queue
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private Stack inStack;
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// Stack to keep track of elements to be removed next in queue
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private Stack outStack;
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/**
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* Constructor
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*/
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public QueueWithStack() {
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this.inStack = new Stack();
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this.outStack = new Stack();
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}
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/**
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* Inserts an element at the rear of the queue
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*
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* @param x element to be added
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*/
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public void insert(Object x) {
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// Insert element into inStack
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this.inStack.push(x);
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}
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/**
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* Remove an element from the front of the queue
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*
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* @return the new front of the queue
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*/
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public Object remove() {
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if(this.outStack.isEmpty()) {
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// Move all elements from inStack to outStack (preserving the order)
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while(!this.inStack.isEmpty()) {
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this.outStack.push( this.inStack.pop() );
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}
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}
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return this.outStack.pop();
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}
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/**
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* Peek at the element from the front of the queue
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*
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* @return the new front of the queue
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*/
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public Object peek() {
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if(this.outStack.isEmpty()) {
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// Move all elements from inStack to outStack (preserving the order)
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while(!this.inStack.isEmpty()) {
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this.outStack.push( this.inStack.pop() );
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}
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}
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return this.outStack.peek();
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}
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/**
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* Returns true if the queue is empty
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*
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* @return true if the queue is empty
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*/
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public boolean isEmpty() {
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return (this.inStack.isEmpty() && this.outStack.isEmpty());
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}
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}
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/**
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* This class is the example for the Queue class
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*
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* @author sahilb2
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*
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*/
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public class QueueUsingTwoStacks {
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/**
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* Main method
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*
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* @param args Command line arguments
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*/
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public static void main(String args[]){
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QueueWithStack myQueue = new QueueWithStack();
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myQueue.insert(1);
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// instack: [(top) 1]
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// outStack: []
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myQueue.insert(2);
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// instack: [(top) 2, 1]
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// outStack: []
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myQueue.insert(3);
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// instack: [(top) 3, 2, 1]
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// outStack: []
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myQueue.insert(4);
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// instack: [(top) 4, 3, 2, 1]
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// outStack: []
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System.out.println(myQueue.isEmpty()); //Will print false
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System.out.println(myQueue.remove()); //Will print 1
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// instack: []
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// outStack: [(top) 2, 3, 4]
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myQueue.insert(5);
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System.out.println(myQueue.peek()); //Will print 2
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// instack: [(top) 5]
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// outStack: [(top) 2, 3, 4]
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myQueue.remove();
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System.out.println(myQueue.peek()); //Will print 3
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// instack: [(top) 5]
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// outStack: [(top) 3, 4]
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myQueue.remove();
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System.out.println(myQueue.peek()); //Will print 4
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// instack: [(top) 5]
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// outStack: [(top) 4]
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myQueue.remove();
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// instack: [(top) 5]
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// outStack: []
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System.out.println(myQueue.peek()); //Will print 5
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// instack: []
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// outStack: [(top) 5]
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myQueue.remove();
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// instack: []
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// outStack: []
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System.out.println(myQueue.isEmpty()); //Will print true
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}
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}
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