leecode刷题（23）-- 合并两个有序链表

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        
        ListNode head = null;
        
        if (l1.val <= l2.val) {
            head = l1;
            head.next = mergeTwoLists(l1.next, l2);
        }
        else {
            head = l2;
            head.next = mergeTwoLists(l1, l2.next);
        }
        
        return head;
    }
}