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

经典插入排序，但要用到三个pointer，其中dummy用来keep track of the head of the linked list，prev每次从dummy开始移动并和cur比较判断是否需要移动，直到找到合适的位置可以插入当前元素。需要记录下cur指向的下一个元素，一次循环结束后cur从next开始

time: O(n^2), space: O(1)

/**
 * 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 dummy = new ListNode(0);
        ListNode prev = dummy;
        ListNode cur = head;
        while(cur != null) {
            ListNode next = cur.next;
            while(prev.next != null && prev.next.val < cur.val) {
                prev = prev.next;
            }
            cur.next = prev.next;
            prev.next = cur;
            prev = dummy;
            cur = next;
        }
        
        return dummy.next;
    }
}