148. Sort List.md

148. Sort List

Problem

Sort a linked list in O(n log n) time using constant space complexity.

tag:

Solution

链表排序

归并排序：

1. 分解： 采用快慢指针找到链表中点，分解为两个子链表
2. 求解： 递归求解子链表
3. 合并： 合并两个有序链表

java

	public ListNode sortList(ListNode head) {
		if (head == null || head.next == null)
			return head;
		ListNode slow = head, fast = head, pre = null;
		while (fast != null && fast.next != null) {
			pre = slow;
			slow = slow.next;
			fast = fast.next.next;
		}
		pre.next = null;
		ListNode l1 = sortList(head), l2 = sortList(slow);
		return merge(l1, l2);
	}

	ListNode merge(ListNode l1, ListNode l2) {
		ListNode dummy = new ListNode(0), p = dummy;
		while (l1 != null && l2 != null) {
			if (l1.val < l2.val) {
				p.next = l1;
				l1 = l1.next;
			} else {
				p.next = l2;
				l2 = l2.next;
			}
			p = p.next;
		}
		if (l1 != null)
			p.next = l1;
		if (l2 != null)
			p.next = l2;
		return dummy.next;
	}

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