Question

How to read an existing file content and then sort them using merge sort into (in java)

1. alphabetical

2. shortest to the longest word

Answer 1

// Java code for above problem

import java.util.*;
import java.io.*;
class Main
{
   public static void main(String args[]) throws Exception
   {
       File file = new File("words.txt");
       BufferedReader br = new BufferedReader(new FileReader(file));
       ArrayList<String> arr=new ArrayList<String>();
       String str="";
       while ((str=br.readLine())!=null)       // read words from file and store them in a list
           arr.add(str);
       System.out.println("Before sorting: "+arr);
       mergeSort(arr,0,arr.size()-1);           // sort the list using merge sort method
       System.out.println("After sorting: "+arr);
   }
   public static void mergeSort(ArrayList<String> arr, int start, int end)
   {
       if(start>=end)
           return;
       int middle=(start+end)/2;
       mergeSort(arr,start,middle);
       mergeSort(arr,middle+1,end);
       merge(arr,start,middle,end);
   }
   public static void merge(ArrayList<String> arr,int start,int middle,int end)
   {
       int n1=middle-start+1;
       int n2=end-middle;
       ArrayList<String> arr1=new ArrayList<String>();
       for(int i=0;i<n1;i++)
           arr1.add(arr.get(start+i));
       ArrayList<String> arr2=new ArrayList<String>();
       for(int i=0;i<n2;i++)
           arr2.add(arr.get(middle+1+i));
       int i=0,j=0,k=start;
       while(i<n1 && j<n2)
       {
           int value=arr1.get(i).compareTo(arr2.get(j));
           if(value<0)
           {  
               arr.set(k,arr1.get(i));
               i++;
           }
           else
           {
               arr.set(k,arr2.get(j));
               j++;
           }
           k++;
       }
       while(i<n1)
       {
           arr.set(k,arr1.get(i));
           i++;
           k++;
       }
       while(j<n2)
       {
           arr.set(k,arr2.get(j));
           j++;
           k++;
       }
   }
}

Sample output:

let "words.txt" has following data:

aaa
abcd
abc
aa
a
zebra
home
my
word
sort

then following will be the output:

Before sorting: [aaa, abcd, abc, aa, a, zebra, home, my, word, sort] After sorting: [a, aa, aaa, abc, abcd, home, my, sort, word, zebra)

// Mention in comments if any mistakes or errors are found. Thank you.