Merge sort

void merge(int arr[], int l, int m, int r)
    {
         // Your code here
         int res[r - l + 1] ;
         int a = l ; 
         int b = m + 1 ; 
         int ind = 0 ; 
         while(a < m + 1 && b < r + 1)
         {
             if(arr[a] < arr[b])
             {
                 res[ind] = arr[a] ;
                 a++ ;
                 ind++;
                 
             }
             else
             {
                 res[ind] = arr[b] ; 
                 b++ ; 
                 ind++ ; 
             }
         }
         if( a < m + 1)
         {
             while( a < m + 1)
             {
                 res[ind] = arr[a] ;
                 a++ ; 
                 ind++ ; 
             }
         }
         else
         {
             while(b < r + 1)
             {
                 res[ind] = arr[b] ; 
                 b++ ; 
                 ind++ ; 
             }
         }
         int k,i  ;
          for(k=0, i=l; k< r-l + 1; k++, i++) {
            arr[i] = res[k];
        }
    }
    public:
    void mergeSort(int arr[], int l, int r)
    {
        //code here
        if( l >= r)
        {
            return ;
        }
        int mid = (l + r) / 2 ;
        mergeSort(arr , l , mid) ;
        mergeSort(arr , mid + 1 , r) ;
        merge(arr , l , mid , r) ;
    }