合并k个链表

class Solution { public ListNode mergeKLists(ListNode[] lists) { if (lists == null || lists.length == 0) return null; return merge(lists, 0, lists.length - 1); }

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

private ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    if (l1 == null) return l2;
    if (l2 == null) return l1;
    if (l1.val < l2.val) {
        l1.next = mergeTwoLists(l1.next, l2);
        return l1;
    } else {
        l2.next = mergeTwoLists(l1,l2.next);
        return l2;
    }
}

}