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784. Letter Case Permutation (Easy)

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.

Solution 1.

// OJ: https://leetcode.com/problems/letter-case-permutation/
// Author: github.com/lzl124631x
// Time: O(2^N)
// Space: O(N)
class Solution {
private:
    vector<string> ans;
    void dfs(string &S, int i) {
        while (i < S.size() && !isalpha(S[i])) ++i;
        if (i == S.size()) {
            ans.push_back(S);
            return;
        }
        dfs(S, i + 1);
        S[i] += ('A' - 'a') * ((S[i] >= 'a' && S[i] <= 'z') ? 1 : -1);
        dfs(S, i + 1);
    }
public:
    vector<string> letterCasePermutation(string S) {
        dfs(S, 0);
        return ans;
    }
};