LeetCode-784

784. Letter Case Permutation

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Given a string S, we can transform every letter individually to be lowercase or uppercase to create another string.  Return a list of all possible strings we could create.

Examples:
Input: S = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]

Input: S = "3z4"
Output: ["3z4", "3Z4"]

Input: S = "12345"
Output: ["12345"]

Note:

• S will be a string with length at most 12.
• S will consist only of letters or digits.

借鉴的很好的思路，一般这种问题我第一想法是递归，但能用迭代解决是最好不过。

迭代中，当遇到字母时，将已处理好的所有字符串再次取出，并按照其它字母不变、仅当前字母分大小写两种形式再次形成新字符串，对应14-19行代码，这里要删除头结点，用LinkedList会更合理一点，最后存入列表。

public class Solution
{
    public IList<string> LetterCasePermutation(string S)
    {
        List<string> res = new List<string>();

        res.Add(S);
        for (int i = 0; i < S.Length; i++)
        {
            if (Char.IsLetter(S[i]))
            {
                for (int j = res.Count() - 1; j >= 0; j--)
                {
                    string str = res[0];
                    res.RemoveAt(0);
                    string left = str.Substring(0, i);
                    string right = str.Substring(i+1);
                    res.Add(left + char.ToLower(S[i]) + right);
                    res.Add(left + char.ToUpper(S[i]) + right);
                }
            }
        }
        return res;
    }
}