21. 合并两个有序链表

function mergeTwoLists(list1: ListNode | null, list2: ListNode | null): ListNode | null {
    let dummy = new ListNode();
    let cur = dummy;

    while(list1&&list2){
        if(list1.val<list2.val){
            cur.next = list1;
            list1 = list1.next;
        }else{
            cur.next = list2;
            list2 = list2.next;
        }
        cur = cur.next;
    }

    if(list1||list2){
        cur.next = list1 || list2;
    }
    return dummy.next;
};