988. Smallest String Starting From Leaf

Given the root of a binary tree, each node has a value from 0 to 25 representing the letters 'a' to 'z': a value of 0 represents 'a', a value of 1 represents 'b', and so on.

Find the lexicographically smallest string that starts at a leaf of this tree and ends at the root.

(As a reminder, any shorter prefix of a string is lexicographically smaller: for example, "ab" is lexicographically smaller than "aba".  A leaf of a node is a node that has no children.)


Example 1:

Input: [0,1,2,3,4,3,4]
Output: "dba"

Example 2:

Input: [25,1,3,1,3,0,2]
Output: "adz"

Example 3:

Input: [2,2,1,null,1,0,null,0]
Output: "abc"



  1. The number of nodes in the given tree will be between 1 and 8500.
  2. Each node in the tree will have a value between 0 and 25.

Rust Solution

struct Solution;
use rustgym_util::*;

trait Preorder {
    fn preorder(&self, cur: &mut Vec<char>, min: &mut String);

impl Preorder for TreeLink {
    fn preorder(&self, cur: &mut Vec<char>, min: &mut String) {
        if let Some(node) = self {
            let node = node.borrow();
            let val = (node.val as u8 + b'a') as char;
            if node.left.is_none() && node.right.is_none() {
                let s: String = cur.iter().rev().copied().collect();
                if min.is_empty() {
                    *min = s;
                } else {
                    if s < *min {
                        *min = s;
            node.left.preorder(cur, min);
            node.right.preorder(cur, min);

impl Solution {
    fn smallest_from_leaf(root: TreeLink) -> String {
        let mut cur: Vec<char> = vec![];
        let mut res: String = "".to_string();
        root.preorder(&mut cur, &mut res);

fn test() {
    let root = tree!(
        tree!(1, tree!(3), tree!(4)),
        tree!(2, tree!(3), tree!(4))
    let res = "dba".to_string();
    assert_eq!(Solution::smallest_from_leaf(root), res);
    let root = tree!(
        tree!(1, tree!(1), tree!(3)),
        tree!(3, tree!(0), tree!(2))
    let res = "adz".to_string();
    assert_eq!(Solution::smallest_from_leaf(root), res);
    let root = tree!(
        tree!(2, None, tree!(1, tree!(0), None)),
        tree!(1, tree!(0), None)
    let res = "abc".to_string();
    assert_eq!(Solution::smallest_from_leaf(root), res);

Having problems with this solution? Click here to submit an issue on github.