☆打卡算法☆LeetCode 17、电话号码的字母组合 算法解析

示例 1：
输入： digits = "23"
输出： ["ad","ae","af","bd","be","bf","cd","ce","cf"]

示例 2：
输入： digits = "2"
输出： ["a","b","c"]

public class Solution {
    public IList<string> LetterCombinations(string digits) {
            IList<string> result = new List<string>();
            if (string.IsNullOrEmpty(digits))
            {
                return result;
            }
            Dictionary<char, string> phoneDic = new Dictionary<char, string>() {
                {'2',"abc" },
                {'3',"def" },
                {'4',"ghi" },
                {'5',"jkl" },
                {'6',"mno" },
                {'7',"pqrs" },
                {'8',"tuv" },
                {'9',"wxyz" },
            };
            if (digits.Length == 1)
            {
                string temp = phoneDic[digits[0]];
                for (int i = 0; i < temp.Length; i++)
                {
                    result.Add(temp[i].ToString());
                }
                return result;
            }
            List<string> temps = new List<string>();
            for(int i = 0; i < digits.Length; i++)
            {
                temps.Add(phoneDic[digits[i]]);
            }
            var tempResult = from m in temps[0]
                         from n in temps[1]
                         select new string("" + m + n);
            for(int i = 2; i < temps.Count; i++)
            {
                string temp = temps[i];
                tempResult = from m in tempResult
                             from n in temp
                             select new string("" + m + n);
            }
            result = tempResult.ToList();
            return result;
    }
}