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public class SmallestStringFromLeaf {

    private static String smallestString;

    public static String smallestFromLeaf(TreeNode root) {
        smallestString = null; // Initialize the smallest string as null
        dfs(root, new StringBuilder());
        return smallestString;
    }

    private static void dfs(TreeNode node, StringBuilder currentString) {
        if (node == null) return;

        // Prepend the current character to the path string
        currentString.insert(0, (char) ('a' + node.val));

        // If it's a leaf node, compare the current string to the smallest string
        if (node.left == null && node.right == null) {
            String currentPath = currentString.toString();
            if (smallestString == null || currentPath.compareTo(smallestString) < 0) {
                smallestString = currentPath;
            }
        }

        // Recurse on the left and right children
        dfs(node.left, currentString);
        dfs(node.right, currentString);

        // Backtrack: Remove the current character from the path
        currentString.deleteCharAt(0);
    }