class Solution {
    public ListNode sortList(ListNode head) {
        if(head==null||head.next==null){
            return head;
        }

        ListNode fast=head.next;
        ListNode slow=head;
        while(fast!=null && fast.next!=null){
            fast=fast.next.next;
            slow=slow.next;
        }
        ListNode head2=slow.next;
        slow.next=null;
        ListNode l1=sortList(head);
        ListNode l2=sortList(head2);
        return merge(l1,l2);
    }

    private ListNode merge(ListNode head1,ListNode head2){
        if(head1==null||head2==null){
            return head1==null?head2:head1;
        }
        ListNode dummyHead = new ListNode(0);
        ListNode temp = dummyHead, temp1 = head1, temp2 = head2;
        while (temp1 != null && temp2 != null) {
            if (temp1.val <= temp2.val) {
                temp.next = temp1;
                temp1 = temp1.next;
            } else {
                temp.next = temp2;
                temp2 = temp2.next;
            }
            temp = temp.next;
        }
        temp.next=temp1==null?temp2:temp1;
        return dummyHead.next;
    }
}

这里，找中间节点时，使用的是这样的方式

        ListNode fast=head.next;
        ListNode slow=head;
        while(fast!=null && fast.next!=null){
            fast=fast.next.next;
            slow=slow.next;
        }
        ListNode head2=slow.next;

我不是很明白 fast=head.next这种写法，我理解的是fast=head也可以，只要把链表分成两部分即可，但是fast=head的时候会报StackOverflowError这是为什么？希望老师解答一下。