How to Sort a Linked List using Insertion Sort in Java

The challenge

Sort a linked list using insertion sort.

A graphical example of insertion sort. The partially 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

Definition for Singly-Linked List

public class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 }

The solution in Java code

class Solution {
    public ListNode insertionSortList(ListNode head) {
        // Create ListNode of the Minimum Integer Value
        ListNode res = new ListNode(Integer.MIN_VALUE);

        // Loop while head exists
        while(head!=null){
            // Create a dummy node
            ListNode dummy = res;

            // Loop while next exists
            while(dummy.next!=null && dummy.next.val < head.val) {
                // Re-set our dummy
                dummy = dummy.next;
            }

            //Swap two nodes
            ListNode next_node = head.next;
            head.next = dummy.next;
            dummy.next = head;
            head = next_node;
        }

        // Return the next item
        return res.next;
    }
}
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