Question

ex3

1. Write a method public static boolean isPalindrome(String input) that uses one or more stacks to determine if a given string is a palindrome. [A palindrome is a string that reads the same forwards and backwards, for example ‘racecar’, ‘civic’]. Make sure that your method works correctly for special cases, if any.

2. What is the big-O complexity of your method in terms of the list size n.

Supplementary Exercise for Programming (Coding)

1. [Stacks] Download and unpack (unzip) the file Stacks.rar. Compile and execute the class StacksTest.java. Use this file structure for completing Exercise 3.

import java.util.*;

public class StacksTest {
   public static void main(String[] args) {
       Stack s = new Stack();
       s.push(new Integer(3));
       s.push(new Integer(5));
       s.push(new String("hi"));
       while(!s.isEmpty()) {
           System.out.print(s.pop() + " ");
       }
      
       s.clear(); //Empty the contents of the stack
      
       System.out.println("\nHere's how I reverse a string: ");
       Scanner k = new Scanner(System.in);
       System.out.print("Enter a string> ");
       String input = k.nextLine();
      
       for(int i = 0; i < input.length(); i++)
           s.push(input.charAt(i) + "");
          
       System.out.println("The reversed string is: ");
       while(!s.isEmpty()) {
           System.out.print(s.pop());
       }
      
       System.out.println();
      
   }
}

------------------------------------------------------------------------------------------------------------------------------

public class Stack {
private java.util.ArrayList pool = new java.util.ArrayList();
public Stack() {
}
  
public Stack(int n) {
pool.ensureCapacity(n);
}
  
public void clear() {
pool.clear();
}
  
public boolean isEmpty() {
return pool.isEmpty();
}
  
public Object topEl() {
if (isEmpty())
throw new java.util.EmptyStackException();
return pool.get(pool.size()-1);
}
  
public Object pop() {
if (isEmpty())
throw new java.util.EmptyStackException();
return pool.remove(pool.size()-1);
}
  
public void push(Object el) {
pool.add(el);
}
  
public String toString() {
return pool.toString();
}
}

Answer 1

import java.util.*;

public class StacksTest {

        static class Stack {
                private ArrayList pool = new ArrayList();

                public Stack() {
                }

                public Stack(int n) {
                        pool.ensureCapacity(n);
                }

                public void clear() {
                        pool.clear();
                }

                public boolean isEmpty() {
                        return pool.isEmpty();
                }

                public Object topEl() {
                        if (isEmpty())
                                throw new java.util.EmptyStackException();
                        return pool.get(pool.size() - 1);
                }

                public Object pop() {
                        if (isEmpty())
                                throw new java.util.EmptyStackException();
                        return pool.remove(pool.size() - 1);
                }

                public void push(Object el) {
                        pool.add(el);
                }

                public String toString() {
                        return pool.toString();
                }
                public static Boolean isPalindrome(String input) {
                    Stack s = new Stack();

                    for (int i = 0; i < input.length(); i++)
                        s.push(input.charAt(i));

                    boolean palindrome = true;
                    int i = 0;
                    while(i < input.length()) {
                        if(input.charAt(i) != (char)s.pop()) {
                            palindrome = false;
                            break;
                        }
                        i++;
                    }
                    return palindrome;
                }
        }

        public static void main(String[] args) {
                Stack s = new Stack();
                s.push(new Integer(3));
                s.push(new Integer(5));
                s.push(new String("hi"));
                while (!s.isEmpty()) {
                        System.out.print(s.pop() + " ");
                }

                s.clear(); // Empty the contents of the stack

                System.out.println("\nHere's how I reverse a string: ");
                Scanner k = new Scanner(System.in);
                System.out.print("Enter a string> ");
                String input = k.nextLine();
                
                for (int i = 0; i < input.length(); i++)
                        s.push(input.charAt(i));

                boolean palindrome = isPalindrome(input);

                System.out.println();

        }
}

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