147. Insertion Sort List

Description

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

My Solution

Source Code

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
let insertionSortList = function(head) {
  let current = head;
  let list = null;
  let previous = null;

  while (current !== null) {
    let next = current.next;
    let insertAfter = list;

    //insert at beginning of list
    if (list === null || current.val <= list.val) {
      current.next = list;
      list = current;
    } else {
      while (insertAfter.next !== null && insertAfter.next.val < current.val) {
        insertAfter = insertAfter.next;
      }
      current.next = insertAfter.next;
      insertAfter.next = current;
    }
    current = next;
  }

  return list;
};

Analysis

There is nothing special about this problem. It's just insertion sort but on a Linked List instead of an array. Implementing it for arrays is very easy but it's a little more complicated with Linked Lists but honestly not that much.