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Merge Sort Implementation in JAVA



Merge Sort  is a technique of sorting data with below steps:-

1) Divide the data recursively until it can not  divide more.

2)  Compare the data and sort it.

3) Merge the data again .




/*
 * Merge Sort is based on the concept below points:-
 * 1) Divide array until it can not be divide.
 * 2) Merge it again in sorted pattern 
 * 
 * 
 */


public class MergeSort {
 
 

 public static int[]  mergeSort(int arr[])
 {
  
  //check for size if it is one then already sorted
  int size=arr.length;
  if(size==1)
  {
   return arr;
  }
  
  //find middle (divide an array into two half)
  int mid=size/2;
  
  //first half
  int[] fhalf=new int[mid];
  //second half
  int[] shalf=new int[size-mid];
  
   // Copy first half of the array
     for (int i = 0; i < mid; i++)
     { fhalf[i] = arr[i];
     }
     // Copy second half of the array
     for (int i = mid; i < size; i++)
     { shalf[i - mid] = arr[i];
     }      
     
     //recursive call fhalf and shalf
     return merge(mergeSort(fhalf), mergeSort(shalf));
  
 }
 
 public static int[] merge(int[] firstHalfSorted,int[] secondHalfSorted)
 {
  // First, create a new array to store the answer. 
  
   int SortedArray[] = new int[firstHalfSorted.length + secondHalfSorted.length];
 

 

    int m = 0;
         int n = 0;
         int count = 0;
         
         System.out.println("first array");
         printArray(firstHalfSorted);
         System.out.println("second array");
         printArray(secondHalfSorted);
         //compare the elements in both arary
         while (m<firstHalfSorted.length && n<secondHalfSorted.length)
         {
             if (firstHalfSorted[m]>secondHalfSorted[n])
             {
                 SortedArray[count]=secondHalfSorted[n];
                 count++;
                 n++;
             }
             else if (firstHalfSorted[m]<secondHalfSorted[n])
             {
                 SortedArray[count]=firstHalfSorted[m];
                 count++;
                 m++;
             }
         }
         
         //append the remaining elements at the end
         if (m!=firstHalfSorted.length)
         {
             while(m<firstHalfSorted.length){
                 SortedArray[count]=firstHalfSorted[m];
                 count++;
                 m++;
             }
         }
         
         //append the remaining elements at the end
         if (n!=secondHalfSorted.length)
         {
             while(n<secondHalfSorted.length){
                 SortedArray[count]=secondHalfSorted[n];
                 count++;
                 n++;
             }
         }
         System.out.println("sorted array");
         printArray(SortedArray);
  // At last return the sorted array 
   return SortedArray;
 }

 
 public static void printArray(int[] arr)
 {
  //System.out.println("printing array");
  for(int i=0;i<arr.length;i++)
  {
   System.out.print(arr[i]+" ");
  }
  System.out.println(" ");
 }
 
 public static void main(String[] args)
 {
  int arr[]={14,33,27,10,25,19,40,45};
  printArray(arr);
  //calling merge function
  arr=mergeSort(arr);
  printArray(arr);
 }

}


Implementation Steps:-


14 33 27 10 25 19 40 45  
first array
14  
second array
33  
sorted array
14 33  
first array
27  
second array
10  
sorted array
10 27  
first array
14 33  
second array
10 27  
sorted array
10 14 27 33  
first array
25  
second array
19  
sorted array
19 25  
first array
40  
second array
45  
sorted array
40 45  
first array
19 25  
second array
40 45  
sorted array
19 25 40 45  
first array
10 14 27 33  
second array
19 25 40 45  
sorted array
10 14 19 25 27 33 40 45  
10 14 19 25 27 33 40 45  

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Example:-
5 1 4 2 8



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