430 扁平化多级双向链表
Node *flatten(Node *head) {
Node *it = head;
stack<Node *> stack1;
while (it) {
if (it->child) {
stack1.push(it->next);
it->next = it->child;
it->child->prev = it;
it->child = nullptr;
} else if (!it->next && !stack1.empty()) {
it->next = stack1.top();
if (it->next)
it->next->prev = it;
stack1.pop();
}
it = it->next;
}
return head;
}