题目：

题解：

class Solution {
public:
    Node* connect(Node* root) {
        if (root == nullptr) {
            return root;
        }
        
        // 从根节点开始
        Node* leftmost = root;
        
        while (leftmost->left != nullptr) {
            
            // 遍历这一层节点组织成的链表，为下一层的节点更新 next 指针
            Node* head = leftmost;
            
            while (head != nullptr) {
                
                // CONNECTION 1
                head->left->next = head->right;
                
                // CONNECTION 2
                if (head->next != nullptr) {
                    head->right->next = head->next->left;
                }
                
                // 指针向后移动
                head = head->next;
            }
            
            // 去下一层的最左的节点
            leftmost = leftmost->left;
        }
        
        return root;
    }
};