Problem:

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Solution:

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    ListNode *head = new ListNode(0);
        ListNode *it = head;
        while (l1 && l2) {
            if (l1->val < l2->val) 
            {
                it->next = l1;
                l1 = l1->next;
            } 
            else 
            {
                it->next = l2;
                l2 = l2->next;
            }
            it = it->next;
        }
        if (l1) it->next = l1;
        if (l2) it->next = l2;
        return head->next;
    }
};

Recursive Solution:

ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) 
{
    if (l1 == NULL) return l2;
    if (l2 == NULL) return l1;
    
    ListNode *ret = NULL;
    
    if (l1->val < l2->val)
    {
        ret = l1;
        ret->next = mergeTwoLists(l1->next, l2);
    }
    else
    {
        ret = l2;
        ret->next = mergeTwoLists(l1, l2->next);
    }
    
    return ret;
}

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