void Main()
{
	var workDir = Path.Combine(Util.MyQueriesFolder, "nunit-work");

	var args = new string[]
	{
		 "-noheader",
		 $"--work={workDir}",
	};

	RunUnitTests(args);
}

// Define other methods and classes here

[Test]
[TestCase(new int[] { 0,0,0,0,0 }, 0, new int[] { 1,2,3,4,5 }, 5, new int[] { 1,2,3,4,5 })] // A6
[TestCase(new int[] { 0 }, 0, new int[] {1 }, 1, new int[] { 1 })] // A5
[TestCase(new int[] { 1, 101,0 }, 2, new int[] { }, 0, new int[] {1,101,0})] // A0
[TestCase(new int[] { 101,102,103,0 }, 3, new int[] { 1 }, 1, new int[] {1,101,102,103})] // A1

public void Merge(int[] nums1, int m, int[] nums2, int n) {
    var mPoint = m-1;
    var nPoint = n-1;
    for(int i = m+n-1; i>=0; i--)
    {
        if(mPoint == -1)
            nums1[i] = nums2[nPoint--];
        else if(nPoint == -1)
            nums1[i] = nums1[mPoint--];
        else if(nums1[mPoint] > nums2[nPoint])
            nums1[i] = nums1[mPoint--];
        else
            nums1[i] = nums2[nPoint--];
    }
}

:56487141
Поглядел на задачу, что вы дали (не подсматривая в ответы) и решил её вот так
public class Solution
{
	public void Merge(int[] nums1, int m, int[] nums2, int n)
	{
		for (int i = m - 1; i >= 0; i--)
			nums1[i + n] = nums1[i];

		int ind1 = n;
		int ind2 = 0;
		int ind = 0;

		while(ind < m+n)
		{
			if (ind1 < (n+m) && ind2 < n)
				nums1[ind++] = nums1[ind1] <= nums2[ind2] ? nums1[ind1++] : nums2[ind2++];

			if (ind1 == (n+m) && ind < m+n) nums1[ind++] = nums2[ind2++];
			if (ind2 == n && ind < m+n) nums1[ind++] = nums1[ind1++];