LeetCode61. 旋转链表

class Solution {
    public ListNode rotateRight(ListNode head, int k) {
        if (head == null || head.next == null || k == 0) return head;
        /**
         * 改进思路1：快慢指针
         */
        /*
        int len = 1;
        ListNode cur = head;
        while (cur.next != null) {
            cur = cur.next;
            len ++;
        }
        k = k % len;
        if (k == 0) return head;
        ListNode fast = head, slow = head;
        for (int i = 0; i < k; i++) {
            fast = fast.next;
        }
        while (fast.next != null) {
            fast = fast.next;
            slow = slow.next;
        }
        ListNode newHead = slow.next;
        slow.next = null;
        fast.next = head;
        return newHead;
        */
        /**
         *  改进思路2：闭合成环后遍历
         */
        // Step1 :求出链表长度
        int len = 1;
        ListNode cur = head;
        while (cur.next != null) {
            cur = cur.next;
            len ++;
        }
        k = k % len;
        if (k == 0) return head;
        // Step2 :闭合成环
        cur.next = head;
        // Step3 :寻找倒数第k个节点的前一个节点
        for (int i = 0; i < len - k; i++) {
            cur = cur.next;
        }
        ListNode newHead = cur.next;
        cur.next = null;
        return newHead;
    }
}