MergeSort.java

// Program that implements Merge sort
// Best case: O(nlogn)
// Average case: O(nlogn)
// Worst case: O(nlogn)

public class MergeSortMain {
 
    static  int  arr[]={100,20,15,30,5,75,40};
 
    public static void main(String args[])
    {
        System.out.println("Array before sorting:");
        printArray(arr,0,arr.length-1);
        System.out.println("-----------------------------");
 
        mergeSort(0,arr.length-1);
 
        System.out.println("-----------------------------");
 
        System.out.println("Array After sorting:");
        printArray(arr,0,arr.length-1);
 
    }
 
    public static void mergeSort(int start,int end)
    {
        int mid=(start+end)/2;
        if(start<end)
        {
            mergeSort(start,mid);
            mergeSort(mid+1,end);
            merge(start,mid,end);
        }
 
    }
 
    private static void merge(int start, int mid, int end) {
        int[] tempArray=new int[arr.length];
        int tempArrayIndex=start;
 
        System.out.print("Before Merging:  ");
        printArray(arr,start,end);
 
        int startIndex=start;
        int midIndex=mid+1;
 
        while(startIndex<=mid && midIndex<=end)
        {
            if(arr[startIndex]< arr[midIndex])
            {
                tempArray[tempArrayIndex++]=arr[startIndex++];
            }
            else
            {
                tempArray[tempArrayIndex++]=arr[midIndex++];
            }
        }
 
        while(startIndex<=mid)
        {
            tempArray[tempArrayIndex++]=arr[startIndex++];
        }
        while(midIndex<=end)
        {
            tempArray[tempArrayIndex++]=arr[midIndex++];
        }
 
        for (int i = start; i <=end; i++) {
            arr[i]=tempArray[i];
        }
 
        System.out.print("After merging:   ");
        printArray(tempArray,start,end);
        System.out.println();
    }
 
    public static void printArray(int arr[],int start,int end)
    {
        for (int i = start; i <=end; i++) {
            System.out.print(arr[i]+" ");
        }
        System.out.println();
    }
}