p1 to m - 1, p2 to n - 1, and p to m + n - 1.p2 >= 0:
a. If p1 >= 0 and nums1[p1] > nums2[p2], place nums1[p1] at nums1[p], decrement p1 and p.
b. Otherwise, place nums2[p2] at nums1[p], decrement p2 and p.nums2 that haven't been copied (when p1 < 0), copy the remaining elements of nums2 to the beginning of nums1.