public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if (l1 == null) {
return l2;
}
if (l2 == null) {
return l1;
}
ListNode fakeHead = new ListNode(-1);
ListNode l3 = fakeHead;
while(l1 != null && l2 != null) {
if (l1.val <= l2.val) {
l3.next = l1;
l1 = l1.next;
l3 = l3.next;
} else {
l3.next = l2;
l2 = l2.next;
l3 = l3.next;
}
}
if (l1 != null) {
l3.next = l1;
}
if (l2 != null) {
l3.next = l1;
}
return fakeHead.next;
}