[LeetCode] 61. Rotate List 旋转链表

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

Example 1:

Input: 1->2->3->4->5->NULL, k = 2
Output: 4->5->1->2->3->NULL
Explanation:
rotate 1 steps to the right: 5->1->2->3->4->NULL
rotate 2 steps to the right: 4->5->1->2->3->NULL

Example 2:

Input: 0->1->2->NULL, k = 4
Output: 2->0->1->NULL
Explanation:
rotate 1 steps to the right: 2->0->1->NULL
rotate 2 steps to the right: 1->2->0->NULL
rotate 3 steps to the right: 0->1->2->NULL
rotate 4 steps to the right: 2->0->1->NULL

与189. Rotate Array 类似，但链表不能通过index来访问，要一步步的走，所以要麻烦一些。

要注意当k大于链表长度时的处理，需要先遍历一遍得到链表长度n，然后k对n取余，用余数来翻转。

Java:

public ListNode rotateRight(ListNode head, int n) {
    if (head==null||head.next==null) return head;
    ListNode dummy=new ListNode(0);
    dummy.next=head;
    ListNode fast=dummy,slow=dummy;

    int i;
    for (i=0;fast.next!=null;i++)//Get the total length 
    	fast=fast.next;
    
    for (int j=i-n%i;j>0;j--) //Get the i-n%i th node
    	slow=slow.next;
    
    fast.next=dummy.next; //Do the rotation
    dummy.next=slow.next;
    slow.next=null;
    
    return dummy.next;
}　　

Python:

# Definition for singly-linked list.
class ListNode:
    def __init__(self, x):
        self.val = x
        self.next = None

    def __repr__(self):
        if self:
            return "{} -> {}".format(self.val, repr(self.next))

class Solution(object):
    def rotateRight(self, head, k):
        """
        :type head: ListNode
        :type k: int
        :rtype: ListNode
        """
        if not head or not head.next:
            return head

        n, cur = 1, head
        while cur.next:
            cur = cur.next
            n += 1
        cur.next = head

        cur, tail = head, cur
        for _ in xrange(n - k % n):
            tail = cur
            cur = cur.next
        tail.next = None

        return cur　

C++:

class Solution {
public:
    ListNode *rotateRight(ListNode *head, int k) {
        if (!head) return NULL;
        int n = 0;
        ListNode *cur = head;
        while (cur) {
            ++n;
            cur = cur->next;
        }
        k %= n;
        ListNode *fast = head, *slow = head;
        for (int i = 0; i < k; ++i) {
            if (fast) fast = fast->next;
        }
        if (!fast) return head;
        while (fast->next) {
            fast = fast->next;
            slow = slow->next;
        }
        fast->next = head;
        fast = slow->next;
        slow->next = NULL;
        return fast;
    }
};

C++:

class Solution {
public:
    ListNode *rotateRight(ListNode *head, int k) {
        if (!head) return NULL;
        int n = 1;
        ListNode *cur = head;
        while (cur->next) {
            ++n;
            cur = cur->next;
        }
        cur->next = head;
        int m = n - k % n;
        for (int i = 0; i < m; ++i) {
            cur = cur->next;
        }
        ListNode *newhead = cur->next;
        cur->next = NULL;
        return newhead;
    }
};

　　

　　

 

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