Problem name: Merge Two Sorted Lists

Problem statement:

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

Example 1:

Input: l1 = [1,2,4], l2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:

Input: l1 = [], l2 = []
Output: []

Solution: Simple solution using recursion.

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* temp;
        if(l1 == NULL) return l2;
        else if(l2 == NULL) return l1;
        
        if(l1->val < l2->val){
            temp = l1;
            temp->next = mergeTwoLists(l1->next, l2);
        }
        else{
            temp = l2;
            temp->next = mergeTwoLists(l1, l2->next);
        }
        return temp;
    }
};