Sort List

思路：
Merge sort.
实现：
先排序mid.next, 在排序head。在排序head之前mid.next = null。

具体merge的时候，循环走完后检查left和right是否为空，如果不为空, 根据链表的特性，直接链接到temp上，不用再一个node一个node的加。

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