合并K个有序链表

合并K个有序链表。
Example:

Input:
[
1->4->5,
1->3->4,
2->6
]
Output: 1->1->2->3->4->4->5->6

分析：
分而治之，将多合并问题分化为二合一问题，每次合并两个list。迭代完成所有链表合并。

public Node mergeKLists(Node[] lists) {
    if (lists == null || lists.length == 0) return null;
    return sort(lists, 0, lists.length - 1);
}

private Node sort(Node[] lists, int lo, int hi) {
    if (lo >= hi) return lists[lo];
    int mid = lo + (hi - lo) / 2;
    Node l1 = sort(lists, lo, mid);
    Node l2 = sort(lists, mid + 1, hi);
    return merge(l1, l2);
}

private Node merge(Node l1, Node l2) {
    if (l1 == null) return l2;
    if (l2 == null) return l1;
    if (l1.val < l2.val) {
        l1.next = merge(l1.next, l2);
        return l1;
    }
    l2.next = merge(l1, l2.next);
    return l2;
}
class Node {
	int val;
	Node next;
	Node(int x) {
    	val = x;
	}
}