"inplace merge sort" with swapping

array1 = [1,6,10,11]
array2 = [5,7,10,20,0,0,0,0]

def swap(arr, arr_index, pointer_index):
    temp = array2[pointer_index]
    array2[pointer_index] = arr[arr_index]
    arr[arr_index] = temp

def solve():
    index1 = len(array1) - 1
    index2 = len(array1) - 1
    for i in range(len(array2) - 1, -1, -1):
        if (array1[index1] > array2[index2]):
            swap(array1, index1, i)
            index1 -= 1
            if (index1 < 0):
                index1 = 0
        else:
            swap(array2, index2, i)
            index2 -= 1
            if (index2 < 0):
                index2 = 0

solve()
print(array2)