lintcode-easy-Binary Tree Paths

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root the root of the binary tree
     * @return all root-to-leaf paths
     */
    public List<String> binaryTreePaths(TreeNode root) {
        // Write your code here
        List<String> result = new ArrayList<String>();
        if(root == null)
            return result;
        
        String str = null;
        helper(str, result, root);
        
        return result;
    }
    
    public void helper(String str, List<String> result, TreeNode root){
        if(root.left == null && root.right == null){
            if(str == null){
                str = String.valueOf(root.val);
            }
            else{
                str = str + "->" + String.valueOf(root.val);
            }
            result.add(str);
            return;
        }
        
        String new_str = null;
        
        if(str == null){
            new_str = String.valueOf(root.val);
        }
        else{
            new_str = str + "->" + String.valueOf(root.val);
        }
        
        if(root.left != null)
            helper(new_str, result, root.left);
        
        if(root.right != null)
            helper(new_str, result, root.right);
        
        return;
        
    }
}