LeetCode23. 合并K个排序链表

/**
 * 合并K有序链表 ==> 对比归并排序
 *
 * @author Ning
 */
public class LeetCode23 {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0)
            return null;
        return mergeKLists(lists, 0, lists.length - 1);
    }

    private ListNode mergeKLists(ListNode[] lists, int left, int right) {
        if (left == right)
            return lists[left];
        int mid = (left + right) / 2;
        ListNode l1 = mergeKLists(lists, left, mid);
        ListNode l2 = mergeKLists(lists, mid + 1, right);
        return mergeTwoLists(l1, l2);
    }

    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode p1 = l1;
        ListNode p2 = l2;
        ListNode prev = dummy;
        while (p1 != null && p2 != null) {
            if (p1.val <= p2.val) {
                prev.next = p1;
                prev = p1;
                p1 = p1.next;
            } else {
                prev.next = p2;
                prev = p2;
                p2 = p2.next;
            }
        }
        prev.next = p1 == null ? p2 : p1;
        return dummy.next;
    }
}

LeetCode148. 排序链表

/**
 * 排序链表
 * @author Ning
 */
public class LeetCode148 {
    public ListNode sortList(ListNode head) {
        if(head == null || head.next == null){
            return head;
        }
        ListNode slow = head;
        ListNode fast = head.next;
        while(fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode temp = slow.next;
        slow.next = null;
        ListNode left = sortList(head);
        ListNode right = sortList(temp);
        ListNode dummy = new ListNode(0);
        ListNode pre = dummy;

        while(left != null && right != null){
            if(left.val < right.val){
                pre.next = left;
                left = left.next;
            }else{
                pre.next = right;
                right = right.next;
            }
            pre = pre.next;
        }
        pre.next = (left == null ? right : left);
        return dummy.next;
    }
}