public class Solution

    {

        public char NextGreatestLetter(char[] letters, char target)

        {

            //a-97  z-122

            var dic = new List<KeyValuePair<char, char>>();

            //key存储当前值，int是下一个值

            for (int i = ; i < letters.Length - ; i++)

            {

                dic.Add(new KeyValuePair<char, char>(letters[i], letters[i + ]));

            }

            dic.Add(new KeyValuePair<char, char>(letters[letters.Length - ], ' '));

            if (target < dic[].Key || target >= dic[dic.Count - ].Key)

            {

                return dic[].Key;

            }

            for (int i = ; i < dic.Count; i++)

            {

                if (target >= dic[i].Key && target < dic[i].Value)

                {

                    return dic[i].Value;

                }

            }

            return dic[].Key;

        }

    }