✅ arrays addition

public class Solution {
    public IList<string> LetterCombinations(string digits) {
       Dictionary<char, char[]> phone = new Dictionary<char,char[]>(){
        {'2', new char[]{'a','b','c'}},
        {'3',new char[]{'d','e','f'}},
        {'4',new char[]{'g','h','i'}},
        {'5',new char[]{'j','k','l'}},
        {'6',new char[]{'m','n','o'}},
        {'7',new char[]{'p','q','r','s'}},
        {'8',new char[]{'t','u','v'}},
        {'9',new char[]{'w','x','y','z'}}};
        
       List<string> combinations = new List<string>(); 
       int length = digits.Length;
       if(digits == null || length == 0) return combinations;
        for(int i = 0; i < length; i++){
            if(length == 1){
                foreach (var letter in phone[digits[i]])
                {
                    combinations.Add(letter.ToString());
                }
            }
                for(int k = 0 ; k < length - 1; k++){
                    
                    for(int j = 0 + k; j < length - 1; j++){
                        int z = 0;
                        combinations.add( phone[digits[k][z]] +=  phone[digits[j][z]]);
                        z++
                        if(z == 2) break;
                    }
                }
                
            
            
        }
        return combinations;
    }
}