61.旋转链表 (Medium)*

class Solution {
public:
    ListNode* rotateRight(ListNode* head, int k) {
        if(head == nullptr) {
            return nullptr;
        }
        ListNode *cur = head;
        int len = 0;
        while(cur != nullptr) {
            len++;
            // 把链表连成环
            if(cur->next == nullptr) {
                cur->next = head;
                break;
            }
            cur = cur->next;
        }
        k = len - k % len - 1;
        cur = head;
        while(k--) {
            cur = cur->next;
        }
        head = cur->next;
        cur->next = nullptr;
        return head;
    }
};