LeetCode-784

Posted 2021-01-01 EasonDongH

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。

 1 public class Solution
 2 {
 3     public IList<string> LetterCasePermutation(string S)
 4     {
 5         List<string> res = new List<string>();
 6 
 7         res.Add(S);
 8         for (int i = 0; i < S.Length; i++)
 9         {
10             if (Char.IsLetter(S[i]))
11             {
12                 for (int j = res.Count() - 1; j >= 0; j--)
13                 {
14                     string str = res[0];
15                     res.RemoveAt(0);
16                     string left = str.Substring(0, i);
17                     string right = str.Substring(i+1);
18                     res.Add(left + char.ToLower(S[i]) + right);
19                     res.Add(left + char.ToUpper(S[i]) + right);
20                 }
21             }
22         }
23         return res;
24     }
25 }