题目：

题解：

class Solution {
public:void handle(Node* &last, Node* &p, Node* &nextStart) {if (last) {last->next = p;} if (!nextStart) {nextStart = p;}last = p;}Node* connect(Node* root) {if (!root) {return nullptr;}Node *start = root;while (start) {Node *last = nullptr, *nextStart = nullptr;for (Node *p = start; p != nullptr; p = p->next) {if (p->left) {handle(last, p->left, nextStart);}if (p->right) {handle(last, p->right, nextStart);}}start = nextStart;}return root;}
};