21. Merge Two Sorted Lists

recursion or iteration

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        
        if(l1.val<l2.val){
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        }else{
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }
}

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode preHead = new ListNode();
        ListNode curr = preHead;
        
        while(l1 != null && l2 != null){
            if(l1.val<l2.val){
                curr.next = l1;
                l1=l1.next;
            }else{
                curr.next = l2;
                l2 = l2.next;
            }
            curr = curr.next;
        }
        curr.next = l1 == null ? l2 : l1;
        return preHead.next;
    }
}