LeetCode 988. Smallest String Starting From Leaf

Problem Statement

use std::rc::Rc;
use std::cell::RefCell;
impl Solution {
    fn dfs(root: Option<&Rc<RefCell<TreeNode>>>, path: String, ans: &mut Vec<String>) {
        let map = vec!['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
        'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'];
        if let Some(n) = root {
            let mut val = map[RefCell::borrow(n).val as usize].to_string();
            val.push_str(&path);

            if RefCell::borrow(&n).left.is_none() && RefCell::borrow(&n).right.is_none() {
                ans.push(val.clone());
                return;
            }
            if !RefCell::borrow(&n).left.is_none() {
                Self::dfs(RefCell::borrow(&n).left.as_ref(), val.clone(), ans);
            }
            if !RefCell::borrow(&n).right.is_none() {
                Self::dfs(RefCell::borrow(&n).right.as_ref(), val.clone(), ans);
            }
        }
    }

    pub fn smallest_from_leaf(root: Option<Rc<RefCell<TreeNode>>>) -> String {
        let mut ans = vec![];
        Self::dfs(Some(&root.unwrap()), "".to_string(), &mut ans);
        ans.sort();
        
        return ans[0].clone();
    }
}