[LeetCode] 148. 排序链表

用归并排序一个链表。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode sortList(ListNode head) {
        return sortList(head, null);
    }

    public ListNode sortList(ListNode head, ListNode tail) {
        if (head == null) {
            return head;
        }
        if (head.next == tail) {
            head.next = null;
            return head;
        }
        ListNode slow = head, fast = head;
        while (fast != tail) {
            slow = slow.next;
            fast = fast.next;
            if (fast != tail) {
                fast = fast.next;
            }
        }
        ListNode mid = slow;
        ListNode list1 = sortList(head, mid);
        ListNode list2 = sortList(mid, tail);
        ListNode sorted = merge(list1, list2);
        return sorted;
    }

    private ListNode merge(ListNode head1, ListNode head2) {
        ListNode ret = new ListNode(0);
        ListNode tmp = ret;
        while (head1 != null && head2 != null) {
            if (head1.val > head2.val) {
                tmp.next = head2;
                head2 = head2.next;
            } else {
                tmp.next = head1;
                head1 = head1.next;
            }
            tmp = tmp.next;
        }
        if (head1 != null) tmp.next = head1;
        if (head2 != null) tmp.next = head2;

        return ret.next;
    }
}