147. Insertion Sort List

Medium

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

解題

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func insertionSortList(head *ListNode) *ListNode {
    dummy := &ListNode{}
    
    cur := dummy

    for head != nil {
        // head : 要插入的值
        t := head.Next // t : 剩下還沒進行 insertion sort 的部分
        cur = dummy

        for cur.Next != nil  && cur.Next.Val <= head.Val {
            cur = cur.Next
        } // 停下來時：cur.Next 比目前 head的值大或是是nil，cur的值與目前 head 的值相等或小於或是是nil

        // 插入
        head.Next = cur.Next
        cur.Next = head
        // 更新要找的值
        head = t
    } 

    return dummy.Next
}