148. Sort List

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

//利用归并排序的思想

class Solution {

    public ListNode sortList(ListNode head) {

        if (head == null || head.next == null)

            return head;

        ListNode prev = null;

        ListNode slow = head;

        ListNode fast = head;

        while (fast != null && fast.next != null ) {

            prev = slow;

            slow = slow.next;

            fast = fast.next.next;

        }

        

        prev.next = null;//将前半段链表设置为null结尾，链表被分成(head,p),(slow,fast)

        

        ListNode l1 = sortList(head);

        ListNode l2 = sortList(slow);

        

        return merge(l1, l2);

    }

    

    private ListNode merge(ListNode l1, ListNode l2) {

        ListNode l = new ListNode(0);//设置辅助节点

        ListNode p = l;

        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;//要把p向前移动

        }

        if (l1 != null)

            p.next = l1;

        if (l2 != null)

            p.next = l2;

        return l.next;

    }

}