Instead of merging from the beginning (which requires extra space or shifts), we merge from the ends of nums1 and nums2 into the end of nums1.

1. Initialize p1 = m - 1, p2 = n - 1, and p = m + n - 1.
2. While p1 >= 0 and p2 >= 0:
   • If nums1[p1] > nums2[p2]:
     • nums1[p] = nums1[p1], p1 -= 1.
   • Else:
     • nums1[p] = nums2[p2], p2 -= 1.
   • p -= 1.
3. If p2 >= 0, fill the remaining nums1 with the rest of nums2.

• Time Complexity: O(M + N).
• Space Complexity: O(1).

def merge(nums1, m, nums2, n):
    p1, p2, p = m - 1, n - 1, m + n - 1
    
    while p1 >= 0 and p2 >= 0:
        if nums1[p1] > nums2[p2]:
            nums1[p] = nums1[p1]
            p1 -= 1
        else:
            nums1[p] = nums2[p2]
            p2 -= 1
        p -= 1
        
    nums1[:p2 + 1] = nums2[:p2 + 1]