(LeetCode)147. Insertion Sort List

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

 以下为评论区中的一种空间复杂度为O(1)的解法：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode insertionSortList(ListNode head) {
        if( head == null ){
			return head;
		}
		
		ListNode helper = new ListNode(0); //新的链表的表头
		ListNode cur = head; //需要插入的节点
		ListNode pre = helper; //节点插入的位置在pre与pre.next之间
		ListNode next = null; //下一个需要插入的节点
        while ( cur != null )
        {
            next = cur.next;
            //找到需要插入的节点的位置
            while ( pre.next != null && pre.next.val < cur.val)
                pre = pre.next;
            //插入节点
            cur.next = pre.next;
            pre.next = cur;
            pre = helper;
            cur = next;
        }
        return helper.next;
            
    }
}