LeetCode 1079. Letter Tile Possibilities

Problem Statement

use std::collections::HashSet;
impl Solution {
    fn dfs(set: &mut HashSet<String>, level: usize, tiles: &String,
           cur: &mut String, used: &mut Vec<bool>) {
        let size = tiles.len();
        if size == level {
            return;
        }
        for i in 0..size {
            if used[i] {
                continue;
            }
            used[i] = true;
            let char = tiles.chars().nth(i).unwrap();
            cur.push(char);
            set.insert(cur.clone());
            Self::dfs(set, level + 1, tiles, cur, used);
            used[i] = false;
            cur.pop();
        }
    }
    pub fn num_tile_possibilities(tiles: String) -> i32 {
        let (mut set, input, mut cur, mut used) =
            (HashSet::new(), tiles.clone(), "".to_string(), vec![false; tiles.len()]);
        Self::dfs(&mut set, 0, &input, &mut cur, &mut used);
        return set.len() as i32;
    }
}