Verify "exist" and "node.Value()".
Set the value if "node.Value()" returns 0.
Write the test cases for "*naiveTrie.Add" and "*naiveTrie.Get".
"*naiveTrie.Get" returns "false" when the node has already created even if the value is zero.
Write the error message for tests of "*naiveTrie.Add" and "*naiveTrie.Get".