988. Smallest String Starting From Leaf

You are given the root of a binary tree where each node has a value in the range [0, 25] representing the letters ‘a’ to ‘z’.

Return 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.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    string res = "";
    void helper(TreeNode* node, string& s) {
        if(!node) return;
        s.push_back(node->val + 'a');
        
        if(!node->left and !node->right) {
            string rs = s;
            reverse(begin(rs), end(rs));
            if(res == "") res = rs;
            else res = min(res, rs);
        } else {
            helper(node->left, s);
            helper(node->right, s);
        }

        s.pop_back();
    }
public:
    string smallestFromLeaf(TreeNode* root) {
        helper(root, string() = "");
        return res;
    }
};