Question

The following is my implementation of merge sort.

private static void mergeSort(int[] a, int low , int high,int[] res)
{
    int mid = (low + high)  /2;
    if (low  < high)
    {
        mergeSort(a,low,mid-1,res);
        mergeSort(a,mid,high-1,res);
        merge(a,low,mid,high,res);

    }
}



   private static void merge(int[] a, int low , int mid , int high,int[] res)
{

    int i = low;
    int j = mid ;

    int k =0;

    while (i < mid && j < high)
        if(a[i] < a[j])
               res[k++] = a[i++];
        else
              res[k++] = a[j++];


    while(i < mid)
        res[k++] = a[i++];

    while(j < high)
        res[k++] =a[j++];
}

When I run this program in the main method, I get the original array printed. Not sure what the problem is. The merge method works when I test is individually though.

public static void main(String[] args)
{
    int[] a = {45,24,53,13,54,45,63,23};
    int[] res = new int[a.length];
    mergeSort(a,0,a.length,res);
    for(int i=0 ; i < res.length ; i++)
    {
       System.out.print(res[i] +",");
    }
}

Output :

 45,24,53,13,54,45,63,23,

I have spent a lot of time looking for the problem. I am not able to fix it.

Answer 1

The answer is as @Mysticial sais, with the exception that the array needs to be copied within the merge method:

private static void mergeSort(int[] a, int low , int high,int[] res)
{
    int mid = (low + high)  /2;
    if (low + 1 < high)
    {
        //  Sort sub-parts
        mergeSort(a,low,mid,res);
        mergeSort(a,mid,high,res);

        //  Merge back to "res"
        merge(a,low,mid,high,res);
    }else{
        res[low] = a[low];
    }
}

private static void merge(int[] a, int low , int mid , int high,int[] res)
{

    int i = low;
    int j = mid;

    int k = low;   //  Use "low" instead of 0.

    while (i < mid && j < high)
        if(a[i] < a[j])
               res[k++] = a[i++];
        else
              res[k++] = a[j++];


    while(i < mid)
        res[k++] = a[i++];

    while(j < high)
        res[k++] =a[j++];

    //  Copy back to "a"
        for (int c = low; c < high; c++){
            a[c] = res[c];
        }

}

At any rate, let it be noted that doing this will overwrite the original array… so you might want to wrap the call to mergeSort to avoid it:

private static int[] mergeSort(int[] a){
    int[] b = new int[a.length];
    int[] tmp = new int[a.length];
    System.arraycopy(a, 0, b, 0, a.length);
    mergeSort(b, 0, b.length, tmp);
    return b;
}

public static void main(String[] args) {
    int[] a = {45, 24, 53, 13, 54, 45, 63, 23};
    int[] res = mergeSort(a);
    for (int i = 0; i < res.length; i++) {
        System.out.print(res[i] + ",");
    }
}

Hope this helps!