[LeetCode] 147. Insertion Sort List_Middle tag: Linked List

Given the head of a singly linked list, sort the list using insertion sort, and return the sorted list's head.

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

思路利用Data structure - Sort & quick sort 小结及leetcode相关题目 里面的insertion sort, 就是将array分成两部分，左边的是sorted， 右边的是unsorted，然后不停将unsorted的element insert到sorted array里面。

T: O(n ^ 2), S: O(1)

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def insertionSortList(self, head: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode()
        cur = head
        while cur:
            pre = dummy

            while pre.next and pre.next.val <= cur.val:
                pre = pre.next
            nextNode = cur.next

            cur.next = pre.next
            pre.next = cur

            cur = nextNode
        return dummy.next