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"

Note:

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.

class Solution {
    String ans = "";

    public void helper(TreeNode node, StringBuilder str) {
        if (node == null)
            return;
        if (node.left == null && node.right == null) {
            char c = (char) ('a' + node.val);
            str.append(c);
            String temp = str.reverse().toString();
            str.reverse(); // Reversing again to bring back
            if (ans.length() == 0 || temp.compareTo(ans) < 0)
                ans = temp;
            str.deleteCharAt(str.length() - 1);
            return;
        }
        char c = (char) ('a' + node.val);
        str.append(c);
        helper(node.left, str);
        helper(node.right, str);
        str.deleteCharAt(str.length() - 1);

    }

    public String smallestFromLeaf(TreeNode root) {
        StringBuilder str = new StringBuilder();
        helper(root, str);
        return ans;
    }
}