Leetcode top100之链表排序

148. 排序链表 - 力扣（LeetCode）

思路比较巧妙

1.使用快慢指针，找到链表的中间节点

2.断开中间连接，对两端递归排序

3.合并两个链表

有快慢指针的技巧和合并链表的技巧

注意点：

快慢指针起点，slow=head，fast=head.next

链表排序前，注意返回头，排完序后，链表头是变化的

    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        ListNode mid = getMidNode(head);
        ListNode temp = mid.next;
        mid.next = null;
        ListNode left = sortList(head);
        ListNode right = sortList(temp);
        return mergeList(left,right);
    }

    private ListNode mergeList(ListNode l1, ListNode l2) {
        ListNode pre = new ListNode(-1);
        ListNode now = pre;
        while (l1 != null && l2 != null) {
            if (l1.val > l2.val) {
                now.next = l2;
                l2 = l2.next;
                now = now.next;
            } else {
                now.next = l1;
                l1 = l1.next;
                now = now.next;
            }
        }
        if (l1 != null) {
            now.next = l1;
        }
        if (l2 != null) {
            now.next = l2;
        }
        return pre.next;
    }

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