leetcode1087

public class Solution
    {

        public string[] Permute(string S)
        {
            List<List<char>> dic = new List<List<char>>();
            int baseindex = 0;
            var temp = S;
            var mask = "";
            while (temp.IndexOf('{') >= 0)
            {
                int begin = temp.IndexOf('{');
                int end = temp.IndexOf('}');
                var ary = new List<char>();
                for (int i = begin + 1; i < end; i++)
                {
                    if (temp[i] != ',')
                    {
                        ary.Add(temp[i]);
                    }
                }
                dic.Add(ary);
                mask = mask + temp.Substring(0, begin) + "#";
                baseindex = end + 1;
                temp = temp.Substring(baseindex);
            }
            mask += S.Substring(S.LastIndexOf('}') + 1);

            var result = LetterCombinations(S, mask, dic);
            return result.OrderBy(x => x).ToArray();
        }

        public IList<string> LetterCombinations(string S, string mask, List<List<char>> dic)
        {
            var combinations = new List<string>();

            if (string.IsNullOrEmpty(S))
                return combinations;

            combinations.Add("");
            foreach (var digits in dic)
            {
                var next = new List<string>();

                foreach (char letter in digits)
                {
                    foreach (string combo in combinations)
                    {
                        var dstr = combo + letter;
                        next.Add(dstr);
                    }
                }
                combinations = next;
            }

            for (int i = 0; i < combinations.Count; i++)
            {
                var cur = combinations[i];
                for (var j = 0; j < mask.Length; j++)
                {
                    if (mask[j] != '#')
                    {
                        cur = cur.Insert(j, mask[j].ToString());
                    }
                }
                combinations[i] = cur;
            }

            return combinations;
        }
    }

思路分析：将字符串的"{}"内的部分和常规字符分开处理。对于大括号内的部分，先将括号内的字符进行“组合”，然后再把常规字符插入到相应的位置。

本题的主要思想和leetcode17: Letter Combinations of a Phone Number是一样的。