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

解題思路：將原鏈表中元素一個一個取出來，在新鏈表中比較進行排序。時間復雜度為O(n2)，是一種效率並不是很高的算法，但是空間復雜度為O(1)，以高時間復雜度換取了低空間復雜度。

方法一（C++）

 1 ListNode* insertionSortList(ListNode* head) {
 
 2         ListNode* dummy=new ListNode(-1),*cur=dummy;
 3         while(head){
 4             ListNode* t=head->next;
 5             cur=dummy;
 6             while(cur->next&&cur->next->val<=head->val)
 7                 cur=cur->next;
 8             head->next=cur->next;
 9             cur->next=head;
10             head=t;
11         }
12         return dummy->next;
13     }

（Java）

 1  public ListNode insertionSortList(ListNode head) {
 2         ListNode dummy=new ListNode(-1),cur=dummy;
 3         while(head!=null){
 4             ListNode t=head.next;
 5             cur=dummy;
 6             while(cur.next!=null&&cur.next.val<=head.val)
 7                 cur=cur.next;
 8             head.next=cur.next;
 9             cur.next=head;
10             head=t;
11         }
12         return dummy.next;
13     }

方法二：不符合題目中的要求，只是完成最基本的鏈表排序功能，借助vector中的sort排序方法。（C++）

 1  ListNode* insertionSortList(ListNode* head) {
 2         vector<int> m;
 3         while(head){
 4             m.push_back(head->val);
 5             head=head->next;
 6         }
 7         sort(m.begin(),m.end());
 8         ListNode* newhead=new ListNode(-1);
 9         while(!m.empty()){
10             ListNode* cur=new ListNode(m.back());
11             m.pop_back();
12             cur->next=newhead->next;
13             newhead->next=cur;
14         }
15         return newhead->next;
16     }

LeetCode 147. Insertion Sort List 鏈表插入排序 C++/Java