Sort a linked list using insertion sort.

这个题我巧妙的设置了一个临时头结点

class Solution {public:    ListNode* insertionSortList(ListNode* head) {        if (head == nullptr)        return head;        ListNode temp(0);        temp.next = head;        head = &temp;        ListNode *cur = head->next;        while (cur->next != nullptr)        {            ListNode *back = head;            while (back->next != cur->next && back->next->val <= cur->next->val)                back = back->next;            if (back->next != cur->next)            {                ListNode *now = cur->next;                cur->next = cur->next->next;                now->next = back->next;                back->next = now;            }            else {                cur = cur->next;            }            }        return temp.next;    }};