def put(self, key, value):
        """
        value will always be non-negative.
        :type key: int
        :type value: int
        :rtype: void
        """
        index = hash(key) % self.size
        if self.nodes[index] is None:
            self.nodes[index] = ListNode(-1, -1)
        prev = find(self.nodes[index], key)
        if prev.next is None:
            prev.next = ListNode(key, value)
        else:
            prev.next.val = value

        value will always be non-negative.
        :type key: int
        :type value: int
        :rtype: void
        
python/706_Design_HashMap.py