merge.c

void merge(int arr[], int left, int mid, int right){

    int len1 = mid - left +1;
    int len2 = right - mid;
    int i;
    int index1 = 0 , index2 = 0;
    // create temp array
    int left_array[len1];
    int right_array[len2];

    //take array to temp array
    for(i=0;i<len1;i++){
        left_array[i] = arr[left+i];
    }
    for(i=0;i<len2;i++){
        right_array[i] = arr[mid+1+i];
    }

    i=left;
    while(index1 < len1 && index2 < len2 ){
        if(left_array[index1] < right_array[index2]){
            arr[i] = left_array[index1];
            index1++;
        }
        else {
            arr[i] = right_array[index2];
            index2++;
        }
        i++;
    }
    // check for other one
    while(index1 < len1){
        arr[i] = left_array[index1];
        i++;
        index1++;
    }
    while(index2 < len2){
        arr[i] = right_array[index2];
        i++;
        index2++;
    }

}