0784. Letter Case Permutation (M)

Letter Case Permutation (M)

题目

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. You can return the output in any order.

Example 1:

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

Example 2:

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

Example 3:

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

Example 4:

Input: S = "0"
Output: ["0"]

Constraints:

• S will be a string with length between 1 and 12.
• S will consist only of letters or digits.

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题意

给定一个字符串，其中的字母可以互换大小写，求所有可能的组合。

思路

DFS。

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代码实现

Java

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

    private void dfs(String S, int index, String cur,  List<String> ans) {
        if (index == S.length()){
            ans.add(cur);
            return; 
        }

        char c = S.charAt(index);
        if (Character.isDigit(c)) {
            dfs(S, index + 1, cur + c, ans);
        } else {
            dfs(S, index + 1, cur + Character.toUpperCase(c), ans);
            dfs(S, index + 1, cur + Character.toLowerCase(c), ans);
        }
    }
}