leetcode : Rotate List

Given a list, rotate the list to the right by k places, where k is non-negative.

For example:
Given 1->2->3->4->5->NULL and k = 2,
return 4->5->1->2->3->NULL.

思路：

（1） 先求出链表的长度length

（2） k%length 防止k大于length

（3） 慢指针指向倒数第k+1，快指针指向最后一个节点

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    
    private int getLength(ListNode head) {
        int length = 0;
        while(head != null) {
            length++;
            head = head.next;
        }
        return length;
    }
    
    public ListNode rotateRight(ListNode head, int k) {
        if(head == null) {
            return head;
        }
        
        ListNode dummy =new ListNode(0);
        dummy.next = head;
        
        ListNode fast = head;
        
        int length = getLength(head);
        k = k % length;
        
        while(k > 0) {
            fast = fast.next;
            k--;
        }
        
        while(fast.next != null) {
            fast = fast.next;
            head = head.next;
        }
        
        // 此时 fast 指向最后一个节点， head 指向倒数第k+1个节点
        fast.next = dummy.next;
        dummy.next = head.next;
        head.next = null;
        
        return dummy.next;
        
        
    }
}