#21. 合并两个有序链表

var mergeTwoLists = function(list1, list2) {
    let l1 = list1
    let l2 = list2
    const dummy = new ListNode(0)
    let p = dummy

    while(l1 && l2) {
        if(l1.val <= l2.val) {
            p.next = new ListNode(l1.val)
            l1 = l1.next
        } else {
            p.next = new ListNode(l2.val)
            l2 = l2.next
        }
        p = p.next
    }

    p.next = l1 === null ? l2 : l1

    return dummy.next
};