61. Rotate List

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

Example:

Given 1->2->3->4->5->NULL and k = 2,

return 4->5->1->2->3->NULL.

思路：
把链表连成环，再在特定的位置断开。注意k可能大于链表的长度。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode rotateRight(ListNode head, int k) {
        if (head == null || k == 0) {  
            return head;  
        }  
  
        int length = 1;  
        ListNode node = head;  
        while (node.next != null) {  
            ++length;  
            node = node.next;  
        }  
  
        node.next = head;  
  
        int m = k % length;  
  
        for (int i = 0; i < length - m; ++i) {  
            node = node.next;  
        }  
  
        head = node.next;  
  
        node.next = null;  
  
        return head;  
    }
}

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode rotateRight(ListNode head, int k) {
        if (head == null || k == 0)
            return head;
        int len = 1;
        
        ListNode node = head;
        while(node.next != null) {
            len++;
            node = node.next;
        }
        
        k = k % len;
        if (k == 0)
            return head;
        
        ListNode ptr = head;
        while(++k < len)
            ptr = ptr.next;
        
        ListNode ans = ptr.next;
        ptr.next = null;
        node.next = head;
        return ans;
    }
}

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} k
 * @return {ListNode}
 */
var rotateRight = function(head, k) {
    if (!head) return head
    const length = lengthOfList(head)
    for (let i = 0; i < (k % length); i++) {
        let newHead = new ListNode(), node = head, prev = head
        while (node.next) {
            prev = node
            node = node.next
        }
        if (length > 0) {
            prev.next = null
            newHead.val = node.val
            newHead.next = head
            head = newHead
        }
    }
    return head
};

const lengthOfList = head => {
    let node = head, length = 0
    while (node) {
        node = node.next
        length++
    }
    return length
}