some code for discuss

test.cpp

while (l1 || l2){
    if (l1.val <= l2.val){
        l3.insert(l1.val)
        l1=l1.next
    }
    else{
        l3.insert(l2.val)
        l2=l2.next
    }
}

while (l1){
    l3.insert(l1.val)
    l1=l1.next
}

while (l2){
    l3.insert(l2.val)
    l2=l2.next
}