class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<>();
        StringBuilder sb = new StringBuilder();
        LinkedList<Integer> list = new LinkedList<>();
        if (root == null) {
            return result;
        }
        helper(root, result, sb);
        return result;  
    }
    public void helper(TreeNode root, List<String> result, StringBuilder sb) {
        if (root == null) {
            return;
        }
    	if (root.left == null && root.right == null) {
    		result.add(sb.append(root.val).toString());
            return;
    	} 
        int len = sb.length();
        if(root.left != null) {
            sb.append(root.val);
            result.add(sb.toString());
            helper(root.left, result, sb);
            sb.setLength(len);//remove the last node value, backtracking
        }
        if(root.right != null) {
            sb.append(root.val);
            result.add(sb.toString());
            helper(root.right, result, sb);
            sb.setLength(len);
        }
    }
}