147. Insertion Sort List

Sort a linked list using insertion sort.

 

A graphical example of insertion sort. The partial sorted list (black) initially contains only the first element in the list.
With each iteration one element (red) is removed from the input data and inserted in-place into the sorted list
 

Algorithm of Insertion Sort:

1. Insertion sort iterates, consuming one input element each repetition, and growing a sorted output list.
2. At each iteration, insertion sort removes one element from the input data, finds the location it belongs within the sorted list, and inserts it there.
3. It repeats until no input elements remain.

Example 1:

Input: 4->2->1->3
Output: 1->2->3->4

Example 2:

Input: -1->5->3->4->0
Output: -1->0->3->4->5

public ListNode insertionSortList(ListNode head) {
        if( head == null ){
            return head;
        }
        
        ListNode helper = new ListNode(0); //new starter of the sorted list
        ListNode cur = head; //the node will be inserted
        ListNode pre = helper; //insert node between pre and pre.next
        ListNode next = null; //the next node will be inserted
        //not the end of input list
        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;
    }