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;
    }
};