对无序链表归并排序

1.代码

public class Solution {
    public ListNode sortList(ListNode head) {
        // 链表结束的条件
        if(head == null || head.next == null){
            return head;
        }

        // 使用归并排序(从底部开始排序，左右两部分，进行链表合并）
        ListNode mid = middleNode(head);
        ListNode rightNode = mid.next;
        mid.next = null;

        ListNode left = sortList(head);
        ListNode right = sortList(rightNode);
        return  merge(left, right);

    }

    //找到链表中间节点
    public ListNode middleNode(ListNode head){

        ListNode slow = head;
        ListNode fast = head;
        while(fast.next != null && fast.next.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    //合并两个有序链表
    public ListNode merge(ListNode head1, ListNode head2) {
        ListNode p1 = head1;
        ListNode p2 = head2;
        ListNode dummyNode = new ListNode(-1);
        ListNode cur = dummyNode;

        while (p1 != null && p2 != null) {
            if (p1.val <= p2.val) {
                cur.next = p1;
                p1 = p1.next;
            } else {
                cur.next = p2;
                p2 = p2.next;
            }
            cur = cur.next;
        }
        if (p1 != null) {
            cur.next = p1;
        } else if (p2 != null) {
            cur.next = p2;
        }
        return dummyNode.next;
    }
}