对链表节点排序，要求时间复杂度为O(nlogn)，空间复杂的为O(1)。
148. Sort List
Sort a linked list in O(n log n) time using constant space complexity.
Example 1:
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Input: 4->2->1->3
Output: 1->2->3->4
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Example 2:
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Input: -1->5->3->4->0
Output: -1->0->3->4->5
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算法：归并排序
在链表上进行排序，由于题目要求时间复杂度O(nlogn)，所以使用归并排序算法。
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public ListNode (ListNode head) {
if (head == null || head.next == null) return head;
ListNode pre = head;
ListNode slow = head;
ListNode fast = head;
while (fast != null && fast.next != null) {
pre = slow;
slow = slow.next;
fast = fast.next.next;
}
fast = slow;
pre.next = null;
ListNode l1 = sortList(head);
ListNode l2 = sortList(fast);
return merge(l1, l2);
}
public ListNode merge(ListNode l1, ListNode l2) {
if (l1 == null) return l2;
if (l2 == null) return l1;
ListNode head = new ListNode(Integer.MIN_VALUE);
ListNode pNode = head;
while (l1 != null && l2 != null) {
if (l1.val < l2.val) {
pNode.next = l1;
pNode = pNode.next;
l1 = l1.next;
} else {
pNode.next = l2;
pNode = pNode.next;
l2 = l2.next;
}
}
if (l1 != null) pNode.next = l1;
if (l2 != null) pNode.next = l2;
return head.next;
}
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时间复杂度：O(nlogn)
空间复杂度：O(1)
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