合并两个有序链表

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
const mergeTwoLists = (list1, list2) => {
    if(!list1){
        return list2
    }else if(!list2){
        return list1
    }else if(list1.val < list2.val){
        list1.next = mergeTwoLists(list1.next, list2)
        return list1
    }else{
        list2.next = mergeTwoLists(list2.next, list1)
        return list2
    }
};