Leetcode -- Insertion Sort List

题目：
Sort a linked list using insertion sort.

分析：
插入排序。

代码1：

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode* insertionSortList(ListNode* head) {        if(!head || !head->next) return head;        ListNode* l1 = head;        ListNode* l1_next;        ListNode* last = l1;        ListNode* l2 = head->next;        ListNode* l2_next;        last -> next = NULL;        l1->next = NULL;        while(l2)        {            l1 = head;            l2_next = l2->next;            while(l1)            {                if(last->val <= l2->val)                {                    last->next = l2;                    last = last->next;                    last->next = NULL;                    break;                }                if(l1->val > l2->val)                {                    l2->next = l1;                    head = l2;                    break;                }                else                {                    if(l1->val <= l2->val && l1->next && l1->next->val > l2->val)                    {                        l1_next = l1->next;                        l1->next = l2;                        l2->next = l1_next;                        break;                    }                    else                    {                        l1 = l1->next;                    }                }             }             l2 = l2_next;        }        return head;    }};

【参考代码：https://leetcode.com/discuss/24663/an-easy-and-clear-way-to-sort-o-1-space】

public ListNode insertionSortList(ListNode head) {        if( head == null ){            return head;        }        ListNode helper = new ListNode(0); //new starter of the sorted list        ListNode cur = head; //the node will be inserted        ListNode pre = helper; //insert node between pre and pre.next        ListNode next = null; //the next node will be inserted        //not the end of input list        while( cur != null ){            next = cur.next;            //find the right place to insert            while( pre.next != null && pre.next.val < cur.val ){                pre = pre.next;            }            //insert between pre and pre.next            cur.next = pre.next;            pre.next = cur;            pre = helper;            cur = next;        }        return helper.next;    }

分析：
思路都是插入排序的思想。代码1比参考代码麻烦的原因是，每次好到插入的地方，就进行插入了。其实，只需要确定插入的地方，然后再进行插入就可以了。没有必要对插入地方进行分类。
其次就是比较的时候，等号的处理。代码1中，是需要考虑这个问题的。