LeetCode刷题（34）-Rotate List

将链表的右边k个节点放到左端，此类问题可以使用快、慢点（差k个节点）的方式实现O(n)复杂度，但是本例存在k大于链表长度的情况，因此先便利求长度。

class Solution(object):    def rotateRight(self, head, k):        """        :type head: ListNode        :type k: int        :rtype: ListNode        """        if not head:            return None        node = head        lenn = 1        while node.next:            node = node.next            lenn += 1        cnt = k % lenn        fast = head        slow = head        i = 0        while  i < cnt:            fast = fast.next            i += 1            if not fast:                return head        while fast.next:            fast = fast.next            slow = slow.next        fast.next = head        dummy = slow.next        slow.next = None        return dummy