public class Solution    {        public char NextGreatestLetter(char[] letters, char target)        {            //a-97  z-122            var dic = new List
     
      
       >();            //key存储当前值，int是下一个值            for (int i = 0; i < letters.Length - 1; i++)            {                dic.Add(new KeyValuePair
       
        (letters[i], letters[i + 1]));            }            dic.Add(new KeyValuePair
        
         (letters[letters.Length - 1], ' '));            if (target < dic[0].Key || target >= dic[dic.Count - 1].Key)            {                return dic[0].Key;            }            for (int i = 0; i < dic.Count; i++)            {                                if (target >= dic[i].Key && target < dic[i].Value)                {                    return dic[i].Value;                }            }            return dic[0].Key;        }    }