#417 · #116 · Populating Next Right Pointers in Each Node(填充每个节点的下一个右侧节点指针)

You are given a perfect binary tree where all leaves are at the same depth and every parent has two children. Each node has an extra next pointer; populate it so each node points to the next node on the same level, with the last node of each level pointing to None. Constraints: the number of nodes is in [0, 2^12 - 1].