Letter Case Permutation

class Solution {
public:
    vector<string> letterCasePermutation(string s) {
        vector<string> out;
        function<void(int)> dfs = [&](int i){
            if (i == (int)s.size()) { out.push_back(s); return; }
            dfs(i + 1);
            if (isalpha((unsigned char)s[i])) {
                s[i] ^= 0x20; // toggle case (ASCII)
                dfs(i + 1);
                s[i] ^= 0x20;
            }
        };
        dfs(0);
        return out;
    }
};

func letterCasePermutation(s string) []string {
    b := []byte(s)
    out := []string{}
    var dfs func(i int)
    dfs = func(i int) {
        if i == len(b) {
            out = append(out, string(b))
            return
        }
        dfs(i + 1)
        if (b[i] >= 'a' && b[i] <= 'z') || (b[i] >= 'A' && b[i] <= 'Z') {
            b[i] ^= 0x20
            dfs(i + 1)
            b[i] ^= 0x20
        }
    }
    dfs(0)
    return out
}

from typing import List

class Solution:
    def letterCasePermutation(self, s: str) -> List[str]:
        b = list(s)
        out: List[str] = []

        def dfs(i: int) -> None:
            if i == len(b):
                out.append(''.join(b))
                return
            dfs(i + 1)
            if b[i].isalpha():
                b[i] = b[i].swapcase()
                dfs(i + 1)
                b[i] = b[i].swapcase()

        dfs(0)
        return out

function letterCasePermutation(s) {
    const b = s.split('');
    const out = [];
    const isAlpha = (c) => /[a-zA-Z]/.test(c);
    const swap = (c) => {
        const code = c.charCodeAt(0);
        return String.fromCharCode(code ^ 0x20);
    };
    const dfs = (i) => {
        if (i === b.length) {
            out.push(b.join(''));
            return;
        }
        dfs(i + 1);
        if (isAlpha(b[i])) {
            b[i] = swap(b[i]);
            dfs(i + 1);
            b[i] = swap(b[i]);
        }
    };
    dfs(0);
    return out;
}

class Solution {
    private char[] b;
    private List<String> out;

    public List<String> letterCasePermutation(String s) {
        b = s.toCharArray();
        out = new ArrayList<>();
        dfs(0);
        return out;
    }

    private void dfs(int i) {
        if (i == b.length) {
            out.add(new String(b));
            return;
        }
        dfs(i + 1);
        if (Character.isLetter(b[i])) {
            b[i] ^= 0x20;
            dfs(i + 1);
            b[i] ^= 0x20;
        }
    }
}

function letterCasePermutation(s: string): string[] {
    const b = s.split('');
    const out: string[] = [];
    const isAlpha = (c: string): boolean => /[a-zA-Z]/.test(c);
    const swap = (c: string): string => String.fromCharCode(c.charCodeAt(0) ^ 0x20);
    const dfs = (i: number): void => {
        if (i === b.length) {
            out.push(b.join(''));
            return;
        }
        dfs(i + 1);
        if (isAlpha(b[i])) {
            b[i] = swap(b[i]);
            dfs(i + 1);
            b[i] = swap(b[i]);
        }
    };
    dfs(0);
    return out;
}