The steps of the insertion sort algorithm:

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.

The following is a graphical example of the insertion sort algorithm. The partially sorted list (black) initially contains only the first element in the list. One element (red) is removed from the input data and inserted in-place into the sorted list with each iteration.

Example 1:

Input: head = [4,2,1,3]
Output: [1,2,3,4]

Example 2:

Input: head = [-1,5,3,4,0]
Output: [-1,0,3,4,5]

Constraints:

• The number of nodes in the list is in the range [1, 5000].
• -5000 <= Node.val <= 5000

Solution: Scan from head

For each node, scan from head of the list to find the insertion position in O(n), and adjust pointers.

Time complexity: O(n²)
Space complexity: O(1)

// Author: Huahua
class Solution {
public:
  ListNode* insertionSortList(ListNode* head) {
    ListNode dummy;
    
    while (head) {
      ListNode* iter = &dummy;
      while (iter->next && head->val > iter->next->val)
        iter = iter->next;
      ListNode* next = head->next;
      head->next = iter->next;
      iter->next = head;
      head = next;
    }
    
    return dummy.next;
  }
};

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