链表，链状结构

61. Rotate List

https://leetcode.com/problems/rotate-list/
除了AC以外，还实现了一些链表的方法，如fromArray， toString等

public class Main {
    public static void main(String[] args) {
        ListNode listNode = new Solution().rotateRight(ListNode.fromArray(new int[]{1, 2, 3, 4, 5}), 2);
        System.out.println(listNode);
    }
}


class ListNode {
    int val;
    ListNode next;

    ListNode(int x) {
        val = x;
    }

    static ListNode fromArray(int[] arr) {
        ListNode head = new ListNode(arr[0]);
        ListNode p = head;
        for (int i = 1; i < arr.length; i++) {
            p.next = new ListNode(arr[i]);
            p = p.next;
        }
        return head;
    }

    @Override
    public String toString() {
        ListNode p = this;
        String str = "";
        while (p != null) {
            str += (p.val);
            if (p.next != null) str += ("->");
            p = p.next;
        }
        return str;
    }
}

class Solution {
    int getLength(ListNode head) {
        int cnt = 0;
        while (head != null) {
            head = head.next;
            cnt++;
        }
        return cnt;

    }

    public ListNode rotateRight(ListNode head, int k) {
        if (head == null) return head;
        int length = getLength(head), i = 0;
        k = k % length;
        if (k == 0) return head;
        k = length - k;
        ListNode pre = head, p = head, end = head;
        while (i < k) {
            pre = p;
            p = p.next;
            end = p;
            i++;
        }
        while (end.next != null)
            end = end.next;
        end.next = head;
        pre.next = null;
        return p;
    }
}