双指针解决链表的问题

1. 背景

双指针解决链表相关的问题，基本都是中等难度

19. 删除链表的倒数第 N 个结点
20. 旋转链表

2. 方案

leetcode 19

public ListNode removeNthFromEnd(ListNode head, int n){
    ListNode second = head;
    ListNode first = head;
    for (int i = 0; i < n; i++) {
        first = first.next;
    }
    if (first == null){
        return head.next;
    }
    while(first.next != null){
        first = first.next;
        second = second.next;
    }
    second.next = second.next.next;
    return head;

}

leetcode 61

public ListNode rotateRight(ListNode head, int k){
    if (head == null || head.next == null){
        return head;
    }
    ListNode temp = head;
    int lens = 1;
    while (temp.next != null){
        lens ++;
        temp = temp.next;
    }
    k = k % lens;
    ListNode first = head;
    ListNode second = head;
    int idx = 0;
    while(first.next != null){
        if (idx ++ < k){
            first = first.next;
        } else {
            first = first.next;
            second = second.next;
        }
    }
    first.next = head;
    head = second.next;
    second.next = null;
    return  head;
}