leetcode: 117. Populating Next Right Pointers in Each Node II

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Difficulty

Medium.

Problem

Given a binary tree

struct TreeLinkNode {
  TreeLinkNode *left;
  TreeLinkNode *right;
  TreeLinkNode *next;
}
Populate each next pointer to point to its next right node. 
If there is no next right node, the next pointer should be set to NULL.

Initially, all next pointers are set to NULL.

Note:

You may only use constant extra space.
Recursive approach is fine, implicit stack space does not count as extra space for this problem.
Example:

Given the following binary tree,

     1
   /  \
  2    3
 / \    \
4   5    7
After calling your function, the tree should look like:

     1 -> NULL
   /  \
  2 -> 3 -> NULL
 / \    \
4-> 5 -> 7 -> NULL

AC

# Definition for binary tree with next pointer.
# class TreeLinkNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
#         self.next = None


class Solution():
    def connect(self, root):
        dummy = TreeLinkNode(-1)
        node = dummy
        while root:
            while root:
                node.next = root.left
                node = node.next or node
                node.next = root.right
                node = node.next or node
                root = root.next
            root, node = dummy.next, dummy