Definisi Singly LinkedList

# Definition of a singly linked list - C++
struct ListNode {
	int val;	
	ListNode *next;
 	ListNode() : val(0), next(nullptr) {}
  	ListNode(int x) : val(x), next(nullptr) {}
 	ListNode(int x, ListNode *next) : val(x), next(next) {}
};
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