update method
Implementation
void update(String? text) {
if (text == null) return;
text = NGram.normalizeVi(text);
NGram gram = NGram();
for (int i = 0; i < text.length; i++) {
String ch = text[i];
gram.addChar(ch);
for (int n = 1; n <= NGram.nGram; n++) {
add(gram.get(n));
}
}
}