java算法-反转链表

/**
 * @author bingo
 * @since 2018/11/22
 */

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    /**
     * 反转链表
     * @param head 链表头部
     * @return 反转后的链表
     */
    public ListNode ReverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }

        ListNode dummy = new ListNode(-1);
        dummy.next = null;
        ListNode p1 = head;
        ListNode p2 = p1.next;
        while (p1 != null) {
            p1.next = dummy.next;
            dummy.next = p1;
            p1 = p2;
            if (p1 == null) {
                break;
            }
            p2 = p1.next;
        }

        return dummy.next;
    }
}

/**
 * @author bingo
 * @since 2018/11/22
 */


/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }

        ListNode next = ReverseList(head.next);
        ListNode cur = next;
        while (cur.next != null) {
            cur = cur.next;
        }
        cur.next = head;
        head.next = null;
        return next;
    }
}