day41（12.22）——leetcode面试经典150

61. 旋转链表

61. 旋转链表

题目：

题解：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode rotateRight(ListNode head, int k) {
        //特判
        if(head == null) {
            return head;
        }
        ListNode cur = head;
        int length = 0; 
        while(cur != null) {
            cur = cur.next;
            length++;
        }
        //k可能大于length，所以k进行取余
        k%=length;
        //如果k==length的时候就是原链表，当k取余后为0的时候也是原链表
        if(k == length || k == 0) {
            return head;
        }
        cur = head;
        int count = 0; 
        ListNode h = new ListNode();
        while(cur.next != null) {
            count++;
            if(count==length-k) {
                h = cur.next;
                cur.next = null;
                cur = h;
            }
            //防止cur.next直接为空，导致cur=cur.next，将cur置为空，再进入循环判断cur.next不为空，就会报空指针异常
            if(cur.next == null) {
                break;
            }
            cur = cur.next; 
        }
        cur.next = head;
        return h;
    }
}