784. Letter Case Permutation

784. Letter Case Permutation

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.

思路： dfs即可，遇到数字跳过，遇到字母，分为两种情况传给子问题。

class Solution { public List letterCasePermutation(String S) { all = new ArrayList<>(); backtrack(S.toCharArray(), 0, ""); return all; } List all; public void backtrack(char[] cs, int pos, String ans) { if (pos == cs.length) { all.add(ans); return; } else { if (Character.isAlphabetic(cs[pos])) { char l = cs[pos]; backtrack(cs, pos + 1, ans + l); if (l >= 'a' && l <= 'z') l = (char) (l - 'a' + 'A'); else if (l >= 'A' && l <= 'Z') l = (char) (l - 'A' + 'a'); backtrack(cs, pos + 1, ans + l); } else { backtrack(cs, pos + 1, ans + cs[pos]); } } }}

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