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. 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.

784. Letter Case Permutation
``````struct Solution;

impl Solution {
fn permutation(s: &[char], n: usize, i: usize, t: &mut String, res: &mut Vec<String>) {
if i == n {
res.push(t.clone());
} else {
if s[i].is_alphabetic() {
let lower: char = s[i].to_ascii_lowercase();
let upper: char = s[i].to_ascii_uppercase();
t.push(lower);
Self::permutation(s, n, i + 1, t, res);
t.pop();
t.push(upper);
Self::permutation(s, n, i + 1, t, res);
t.pop();
} else {
t.push(s[i]);
Self::permutation(s, n, i + 1, t, res);
t.pop();
}
}
}
fn letter_case_permutation(s: String) -> Vec<String> {
let n = s.len();
let s: Vec<char> = s.chars().collect();
let mut res: Vec<String> = vec![];
let mut t: String = "".to_string();
Self::permutation(&s, n, 0, &mut t, &mut res);
res
}
}

#[test]
fn test() {
let s: String = "a1b2".to_string();
let res: Vec<String> = vec_string!["a1b2", "a1B2", "A1b2", "A1B2"];
assert_eq!(Solution::letter_case_permutation(s), res);
}
``````