147. Insertion Sort List

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode insertionSortList(ListNode head) {
        if(head==null) return head;//corner case:if the List is null;
        ListNode helper = new ListNode(0);
        ListNode pre = helper,//insert between pre and pre.next
        cur = head,//the node will be inserted
        next = null;//the next node will be valued by cur
        while(cur!=null){
            next = cur.next;
            // find the right place to insert
            while(pre.next!=null&&pre.next.val<cur.val){
                pre=  pre.next;
            }
            // insert between pre and pre.next
            cur.next = pre.next;
            pre.next = cur;
            pre = helper;
            cur = next;
        }
        return helper.next;
    }
}
// as for the run time complexity,for the worst case, it will cost O(n^2) time. As for the space complexity,it will take O(n)