LeetCode(147): Insertion Sort List

public ListNode insertionSortList(ListNode head) {
        if(head==null||head.next==null) return head;
        
        ListNode p = head.next;   //指向第2个结点
        ListNode q= head;        //指向头结点
        ListNode preq= null;
        ListNode pNext = null;
        head.next = null;  //将head.next置空，使头结点和链表断开
        while(p!=null){   //用p进行遍历
            q = head;    //用q保存头结点
            pNext = p.next; //pNext保存p.next
            while(q!=null && p.val>q.val){  //在新的链表中查找小于等于p.val的节点
                preq = q;
                q=q.next;
            }
            if(q==head){  //如果是头结点，则将其插入到头结点
                p.next = q; 
                head = p; 
            }else{ //否则进行普通的插入操作
                p.next = preq.next;
                preq.next = p;
            }
            p=pNext;
            
        }
        return head;
        
    }