147. Insertion Sort List

Sort a linked list using insertion sort.

public class Solution {

    public ListNode insertionSortList(ListNode head) {

 

        ListNode root = new ListNode(0); // 头结点

        root.next = head;

        ListNode p = head;

        ListNode q;

        ListNode r;

//例如6->4->3->7->1->2

        while (p != null && p.next != null) {

            if (p.val <= p.next.val) {//如果此时p等于3,3<7,符合升序，p就指向7

                p = p.next;

            }else { //例如6大于4，则把4和6互换，另q等于4，所以6就指向了3(q.next)

                q = p.next;

                p.next = q.next;

                

                r = root;                                

                while (r.next.val <= q.val) { //此时r.next是6,6大于4，不执行循环

                    r = r.next;//最后进行到0->1->3->4->6->7->2，p等于7，q等于2，则7指向null(q.next)，此时1小于2(r.next.val < q.val)，所以另r等于1，再往下走，则让2->3, 1->2

                }

 

                q.next = r.next; //使得4指向6

                r.next = q; //root指向4，完成4->6->3

            }

        }

 

        return root.next;

    }

}