784. Letter Case Permutation

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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 between 1 and 12.
S will consist only of letters or digits.

图解

字符的大小写转换:

char a = ‘a’;// a= ‘A’

char s = a ^= 1 << 5;

使用深度优先搜索

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public class LetterCasePermutation {

   public List<String> letterCasePermutation(String S) {
      List<String> ans = new ArrayList<>();
      dfs(S.toCharArray(), 0, ans);
      return ans;
   }

   private void dfs(char[] S, int i, List<String> ans) {
      if (i == S.length) {
         ans.add(new String(S));
         return;
      }
      dfs(S, i + 1, ans);
      if (!Character.isLetter(S[i]))
         return;
      S[i] ^= 1 << 5;
      dfs(S, i + 1, ans);
      // 不需要回溯的话，可以只 toggle 一次
//    S[i] ^= 1 << 5;
   }

}