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In case that array 2 is long enough to accommodate both of the arrays then its is filled its size minus array 1's size. (len2-len1 = biggest number on array 2)

int* mergeSort(int* arr1, int len1, int* arr2, int len2)
{
	int* runner = arr2 + len2 -1; //will mark the cell we want to place the bigger number
	int realLen2 = len2 - len1; //the actual length of array2- the values for the merge
	
	int* endOfArr1 = arr1 + len1 -1; //the end of array1 which is the biggest number.
	int* endOfArr2 = arr2 + realLen2 -1; //the biggest number on array2
	
	//run until you reach the end of one of the arrays
	//it will always be completely sorted by then.
	while(len1 > 0 && realLen2 > 0)
	{
		if(*endOfArr1 >= *endOfArr2)
		{
			*runner = *endOfArr1;
			endOfArr1--;
			len1--;
		}
		else
		{
			*runner = *endOfArr2;
			endOfArr2--;
			realLen2--;
		}
		runner--;
	}
	
	//in case there are leftovers in array1 merge them (in case all array1 is smaller)
	while(len1 > 0)
	{
		*runner = *endOfArr1;
		endOfArr1--;
		len1--;
		runner--;
	}
	return arr2;
}