Bullet Brain.

The story is set in the 1960s Hong Kong. The sincere Ko Tai Hei smuggles to Hong Kong, where his father's good friend Kam Lo Yik recommended him to join the police force. One time, he had an accident and got shot in the head. Pieces of the bullet went into his brain cells and oddly increased his IQ significantly. Since then, Hei turned into an intelligent detective capable of cracking down strange cases. However, his personality changed as well, turning him into a cold-hearted person who will do things by unscrupulous means.