Merge List

class Solution {
public:
	ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) 
  {
		if(l1 == NULL)
        {
			return l2;
		}
		
		if(l2 == NULL)
        {
			return l1;
		} 
		
		// if value pointend by l1 pointer is less than equal to value pointed by l2 pointer
		// we wall call recursively l1 -> next and whole l2 list.
		if(l1 -> val <= l2 -> val)
        {
			l1 -> next = mergeTwoLists(l1 -> next, l2);
			return l1;
		}
		// we will call recursive l1 whole list and l2 -> next
		else
        {
			l2 -> next = mergeTwoLists(l1, l2 -> next);
			return l2;            
		}
	}
};