private void printLeftSide(Node root){
        if(root == null || (root.left == null && root.right == null)){
            return;
        }
        if(root.left != null){
            printLeftSide(root.left);
        }else{
            printRightSide(root.right);
        }
        System.out.println(root.data);
    }
http://www.geeksforgeeks.org/boundary-traversal-of-binary-tree/
Test cases
All left children
All right children
Full tree
Complete tree
src/com/interview/tree/BoundaryTraversal.java