A classic subroutine of merge sort. Just merge the elements from back to forth. Keep a pointer for the merged position of the element and two other pointers for elements in nums1 and nums2 respectively.

The code is as follows.

1 class Solution { 2 public: 3     void merge(vector
     
      & nums1, int m, vector
      
       & nums2, int n) { 4         int p = m + n - 1, p1 = m - 1, p2 = n - 1; 5         while (p1 >= 0 && p2 >= 0) { 6             if (nums1[p1] >= nums2[p2]) 7                 nums1[p--] = nums1[p1--]; 8             else nums1[p--] = nums2[p2--]; 9         }10         while (p2 >= 0)11             nums1[p--] = nums2[p2--];12     }13 };