np.py

class Solution:
    def connect(self, root):
        self.do_connect(root, None)

    def do_connect(self, node, right_sib):
        if not node:
            return
        node.next = right_sib if right_sib else None
        if node.left:
            self.do_connect(node.left, right_sib=node.right)
        if node.right:
            if right_sib:
                self.do_connect(node.right, right_sib=right_sib.left)
            else:
                self.do_connect(node.right, right_sib=None)

    def build_tree(self, num_levels, val):
        root = TreeLinkNode(val)
        if num_levels > 1:
            root.left = self.build_tree(num_levels - 1, val*2 + 1)
            root.right = self.build_tree(num_levels - 1, val*2 + 2)
        return root

class TreeLinkNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None
        self.next = None