*LeetCode-Binary Tree Paths

いくつかのcorner caseは、例えば1つのノード、例えばノードがないroot自身はpathとは言えないからです.
root+->を使うのは->rootより便利です

public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> list = new ArrayList<String> ();
        if ( root == null )
            return list;
        helper ( list, "", root);
        return list;
    }
    public void helper ( List<String> list, String str, TreeNode cur ){
        if ( cur.left == null && cur.right == null ){
            str += Integer.toString( cur.val );
            list.add ( str );
            return;
        }
        str = str + Integer.toString( cur.val ) + "->";
        if ( cur.left != null )
            helper ( list, str, cur.left );
        if ( cur.right != null )
            helper ( list, str, cur.right );
    }
}