21-合并两个有序链表

public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
    //虚拟头部
    ListNode root = new ListNode(), cur = root;
    //先合并公共长度部分
    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;
    }
    //剩下的直接接上
    cur.next = (l1 == null) ? l2 : l1;
    return root.next;
}