问题描述

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

对链表进行排序，要求时间复杂度为O(nlog(n))，空间复杂度为O(1)

举个例子：

Input: 4->2->1->3
Output: 1->2->3->4

解法1

Thanks 花花酱 LeetCode 148. Sort List - 刷题找工作 EP211{:target=”_blank”}

先提供一个时间复杂度为O(nlog(n))、空间复杂度为O(log(n))的版本：

主要是利用归并思想：

来看下实现：

class Solution {
    public ListNode sortList(ListNode head) {

        if (head == null || head.next == null){
            return head;
        }
        ListNode fast = head;
        ListNode slow = head;
        ListNode pre  = head;

        while (fast != null && fast.next != null){
            pre  = slow;
            fast = fast.next.next;
            slow = slow.next;
        }

        pre.next = null;
        fast = head;

        ListNode l1 = sortList(fast);
        ListNode l2 = sortList(slow);
        return merge(l1, l2);
    }

    private ListNode merge(ListNode l1, ListNode l2){
        ListNode dummy = new ListNode(-1);
        ListNode cur = dummy;

        while (l1 != null && l2 != null){
            if (l1.val <= l2.val){
                cur.next = l1;
                cur = l1;
                l1 = l1.next;
            } else {
                cur.next = l2;
                cur = l2;
                l2 = l2.next;
            }
        }

        if (l1 != null) cur.next = l1;
        if (l2 != null) cur.next = l2;
        return dummy.next;
    }
}

Enjoy it !