03_填充每个节点的下一个右侧节点指针

"""
# Definition for a Node.
class Node(object):
    def __init__(self, val, left, right, next):
        self.val = val
        self.left = left
        self.right = right
        self.next = next
"""


class Solution(object):
    def connect(self, root):
        """
        :type root: Node
        :rtype: Node
        """
        def helper(root):
            if not root:
                return
            if not root.left:
                return
            root.left.next = root.right
            if root.next:
                root.right.next = root.next.left
            helper(root.left)
            helper(root.right)
        helper(root)
        return root