【Leetcode】Insertion Sort List

题目链接：https://leetcode.com/problems/insertion-sort-list/

题目：

Sort a linked list using insertion sort.

思路：

头插法。用头结点可以简化插入链表时候的操作，因为要考虑插入链表中间和表头两种情况，插入表头时，head就要更新，还要判断pre指针是否为空

算法：

public ListNode insertSortList(ListNode head, ListNode t) {
ListNode p = head.next, pre = head;// p t pre
while (p != null && p.val < t.val) {
pre = p;
p = p.next;
}
t.next = p;
pre.next = t;
return head;
}

public ListNode insertionSortList(ListNode head) {
ListNode p = head, tmp = p;
ListNode newHead = new ListNode(0); //头结点
while (p != null) {
tmp = p;
p = p.next;
newHead = insertSortList(newHead, tmp);
}
return newHead.next;
}