smallestFromLeaf.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {string}
 */
var smallestFromLeaf = function(root) {
    // We need to initialize the smallest string as a large value
    let smallest = "~";

    const dfs = (node, str) => {
        // If the node is null, we return
        if (!node) return;

        // We add the current node's value (converted to a character) to the start of the string
        str = String.fromCharCode(node.val + 97) + str;

        // If the node is a leaf (it has no children), we compare the string with the smallest string found so far
        if (!node.left && !node.right) {
            if (str < smallest) smallest = str;
        }

        // We continue the depth-first search on the left and right children
        dfs(node.left, str);
        dfs(node.right, str);
    };

    // We start the depth-first search from the root
    dfs(root, "");

    // We return the smallest string found
    return smallest;
};