147. Insertion Sort List #

Problem #

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:

Insertion sort iterates, consuming one input element each repetition, and growing a sorted output list. 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. 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

Summary #

Insertion sort on a linked list

Solution Approach #

Just do as the problem requires.

Code #

package leetcode

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func insertionSortList(head *ListNode) *ListNode {
	if head == nil {
		return head
	}
	newHead := &ListNode{Val: 0, Next: nil} // Do not initialize this to point directly to head, so the loop below can handle cases uniformly.
	cur, pre := head, newHead
	for cur != nil {
		next := cur.Next
		for pre.Next != nil && pre.Next.Val < cur.Val {
			pre = pre.Next
		}
		cur.Next = pre.Next
		pre.Next = cur
		pre = newHead // Reset, start from the beginning.
		cur = next
	}
	return newHead.Next
}