String has same characters at same position

You are given an arrty of S consisting of N strings. Every string is of the same length M. YOUr task is to find a pair of string in array S, such that there exists a position in which both of the strins have the same letter. Both the index in array S and the position in the string are numbered from zero. For example given S=["Abc","aca","abe"]. String "abc" and string "dba" have same character "b" in position 1

public class Solution
{
    public int[] solution(string[] S)
    {
        //ensure string is not null here
        if (S?.Length > 0)
        {
            Dictionary<char, Dictionary<int, int>> dict = [];

            for (int i = 0; i < S.Length; i++)
            {
                for (int j = 0; j < S[i].Length; j++)
                {
                    if (!dict.ContainsKey(S[i][j]))
                    {
                        dict[S[i][j]] = [];
                    }
                    if (dict[S[i][j]].ContainsKey(j))
                    {
                        return [dict[S[i][j]][j], i, j];
                    }
                    else
                    {
                        dict[S[i][j]][j] = i;
                    }
                }
            }
        }
        return [];
    }

}