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You are given a perfect binary tree where all leaves are on the same level, and every parent has two children. The binary tree has the following definition:

struct Node {  int val;  Node *left;  Node *right;  Node *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.

注意保存下一层最左面的点，也就是next。

剩下就是判断当前点cur是否是这一层最右面的点。

总体还是比较简单的。

class Solution:    def connect(self, root: 'Node') -> 'Node':        if not root:return root        cur = root        next = root.left                while cur.left:            cur.left.next = cur.right            if cur.next:                cur.right.next=cur.next.left                cur=cur.next            else:                cur=next                next=cur.left        return root

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