21 合并两个有序链表

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

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    ListNode head = new ListNode(0);
    ListNode cur = head;
    while (l1 != null && l2 != null) {
        if (l1.val <= l2.val) {
            cur.next = l1;
            l1 = l1.next;
        } else {
            cur.next = l2;
            l2 = l2.next;
        }
        cur = cur.next;
    }
    //处理最后的，另一个链表无论还剩多少个都是排好序的并且比当前大直接拼在当前之后就好
    cur.next = l1 == null ? l2 : l1;
    return cur.next;
}

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    if (l1 == null) return l2;
    else if (l2 == null) return l1;
    //谁是头
    if (l1.val < l2.val) {
        l1.next = mergeTwoLists(l1.next, l2);
        return l1;
    } else {
        l2.next = mergeTwoLists(l1, l2.next);
        return l2;
    }
}