Question

Complete the following methods:
secondlargest(): returns the second largest in the list without sorting. Assumes list
has at least 2 elements.
palindrome(): checks if list elements are palindrome, i.e. list elements from left to
right and right to left, are identical.


Code:

import java.util.*;
import java.io.*;

public class xxxxxL1 {
   // class Variables
    private int n, count, arr[];
   PrintStream prt = System.out;
   // xxxxxL1 Constructor
   xxxxxL1(int k){
      count = 0;
      n = k;
      //   Allocate Space for array         
      arr = new int[n+1];// index 0 is not used
      prt.printf("\tCreating array of size %2d:",n);
   } // end constructor
  
   //prints list formatted 10 elements per line.
   private void printlist(){
      int i;
      prt.print("\n\tList contents:\n\t");
      for (i = 1; i <= count; i++){
       prt.printf("%2d, ", arr[i]);
       if (i % 10 == 0) prt.printf("\n\t");
      }
    } // end printlist  
  
   // Return second largest in the list.
   // Assume list has 2 or more elements.
    private int secondlargest(){
       return 0;
    } // end secondlargest
      
    // deletes x from list, if exist.
    private int delete(int x){
      int i, j;
      if (count == 0)
           return 0; // invalid position
   //shift array elements from position p to right
      for (i = 1 ; i <= count ; i++)
       if (arr[i] == x){
       for (j = i ; j < count ; j++)
             arr[j] = arr[j+1];
       count--;
       return 1;
       }
      return 1;
    } //end delete

   //checks if list elements are palindrome, i.e.
   //list elements from left to right and right to
   //left, are identical.
    private int palindrome(){
       return 0;
    } // end palindrome

   //insert x at position p, in list
   //for successful insertion: list should not be
   // full(count=n) and 1 <= p <= count+1
    private int insert(int x, int p){
      int i;
      if (count == n || p < 1 || p > count+1)
           return 0; // invalid position
   //shift array elements from position p to right
      for (i = count ; i >= p ; i--)
       arr[i+1] = arr[i];
      // end for
              
      arr[p] = x; // insert x at position p
      count++; // increment no. of elements.
      return 1; // successful insertion
   } // end insert

   public static void main(String args[]) throws Exception{
      int j, m, k, p, s, x;
      try{
       // open input file
       Scanner inf = new Scanner(System.in);
        // read list size      
       k = inf.nextInt();
       // Create a List of type Integer of size k
       xxxxxL1 lst = new xxxxxL1(k);          

       //read no. of elements to insert
       m = inf.nextInt();
       //Insert m elements in the list
        for(j = 1; j <= m; j++){
           x = inf.nextInt();   // read x
           p = inf.nextInt(); // read position
          //insert x at position p
          s = lst.insert(x, p);
           //if (s == 1)System.out.printf(" Successful insertion.");  
           //else               System.out.printf(" Can not insert.");
         } // end for
      
       //prints list formatted 10 elements per line.
       lst.printlist();

       //print second largest
       //p = lst.secondlargest();
         //System.out.printf("\n\t2nd largest = %d.", p);

        //read no. of elements to delete from list
        k = inf.nextInt();
       //Delete k elements from the lis
        for(j = 1; j <= k; j++){
           x = inf.nextInt();   // read next int
           p = lst.delete(x);   //delete x
           //if (p ==1) System.out.printf(": deleted.");
           //else System.out.printf(": not found.");
        }// end for

       //prints list formatted 10 elements per line.
       lst.printlist();
      
       // close input file       
       inf.close();
       } catch (Exception e) {
        System.out.print("\nException " + e + "\n");}
    } // end main
   } // end class xxxxxL1

Answer 1

I have made a program to find 2nd largest and check if it is palindrome. It acts on a list. Please incorporate the same in your code.

2nd Largest

 private int secondlargest() {
     int largest = 0, secondLargest = 0;
     for (int i = 0; i < arr.length; i++) {
         if (i == 0) {
             largest = arr[i]; // initially consider 1st element as largest
         } else {
             if (largest < arr[i]) {       // if 2nd element is larger than 1st, secondLargest becomes the previous largest and currect largest is the ith element
                 secondLargest = largest;
                 largest = arr[i];
             } else {                      
                 if (secondLargest < arr[i]) {      // if ith element is lesser than largest, we compare the secondLargest element with ith element and change secondLargest to ith element if ith element is greater
                     secondLargest = arr[i];
                 }
             }
         }
     }
     return secondLargest;   //last value of secondLargesr after we are done with traversing is 2nd largest value of list
 }

INPUT/OUTPUT

arr = {1 , 2 , 3 , 5}
secondLargest is 3


Palindrome -> As it checks, the return type I have changed to boolean. You can use int also. 1 for palindrome and 0 for not palindrome.

  private boolean palindrome() {
      int length = arr.length;
      if (length % 2 == 0) {
          //if length is even
          for (int i = 0; i < (length) / 2; i++) { // if length is odd, we compare first and last. 2nd and 2nd last. 3rd and 3rd last and so on..... The middle element is NOT common while traversing from both sides. SO, comparing first half and 2nd half.
              if (arr[i] != arr[length - 1 - i]) {
                  return false;
              }
          }
      } else {
          //if length is odd
          for (int i = 0; i < (length - 1) / 2; i++) { // if length is odd, we compare first and last. 2nd and 2nd last. 3rd and 3rd last and so on..... The middle element is common while traversing from both sides
              if (arr[i] != arr[length - 1 - i]) {
                  return false;
              }
          }

      }
      return true;
  }

INPUT/OUTPUT

arr = {1 , 2 , 3 , 0 , 0 , 3 , 2 , 1} -> TRUE
arr = { 1 , 2 , 3 , 2 , 1} -> TRUE
arr={1 ,2 ,3 ,1 , 2} -> FALSE






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