Merge Sort Implementation in C++

Merge Sort is a technique in which we use the algorithm of divide and conquer.
The input array is first divided into smaller sub-arrays, which are sorted in turn and again merged to get the original array in a sorted manner.

Lets see how it works :

This is our original array

24,56,12,34,3,78,32,9

Now we divide it into two parts :

24,56,12,34           3,78,32,9

Further these parts are sub divided ,

24,56      12,34           3,78    32,9

Then each part is sorted individually

24,56      12,34           3,78    9,32 

Again these sub parts are merged in sorted manner

24,56  +   12,34           3,78   +   9,32 

12,24,34,56      +       3,9,32,78

3,9,12,24,32,34,56,78

#include <iostream>

using namespace std;

void merge(int*,int*,int,int,int);
void mergesort(int *a, int*b, int low, int high)
{
    int pivot;
    if(low<high)
    {
        pivot=(low+high)/2;
        mergesort(a,b,low,pivot);
        mergesort(a,b,pivot+1,high);
        merge(a,b,low,pivot,high);
    }
}
void merge(int *a, int *b, int low, int pivot, int high)
{
    int h,i,j,k;
    h=low;
    i=low;
    j=pivot+1;

    while((h<=pivot)&&(j<=high))
    {
        if(a[h]<=a[j])
        {
            b[i]=a[h];
            h++;
        }
        else
        {
            b[i]=a[j];
            j++;
        }
        i++;
    }
    if(h>pivot)
    {
        for(k=j; k<=high; k++)
        {
            b[i]=a[k];
            i++;
        }
    }
    else
    {
        for(k=h; k<=pivot; k++)
        {
            b[i]=a[k];
            i++;
        }
    }
    for(k=low; k<=high; k++) a[k]=b[k];
}

int main()
{
    int a[] = {12,10,43,23,-78,45,123,56,98,41,90,24};
    int num;

    num = sizeof(a)/sizeof(int);

    int b[num];

    mergesort(a,b,0,num-1);

    for(int i=0; i<num; i++)
        cout<<a[i]<<" ";
    cout<<endl;
}