Merge sorted arrays (in-place)

class Solution:
    def merge(self, nums1: List[int], m: int, nums2: List[int], n: int) -> None:
        """
        Do not return anything, modify nums1 in-place instead.
        """

        first = m - 1
        second = n - 1
        third = m + n - 1

        while(first >= 0 and second >=0):
            if nums1[first] >= nums2[second]:
                nums1[third] = nums1[first]
                third -= 1
                first -= 1
            else:
                nums1[third] = nums2[second]
                third -= 1
                second -= 1

        while(second >= 0):
            nums1[third] = nums2[second]
            second -= 1
            # made the mistake of not decreasing the third pointer
            third -= 1