合并两个有序链表

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
    let res = new ListNode(-1);
    let cur = res;
    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;
    }
    cur.next = (l1 === null) ? l2 : l1;
    return res.next;
};