Two worlds will collide when Donald Trump meets Kim Jong Un in Singapore. The 72-year-old billionaire known for his round-the-clock Twitter outbursts will sit face-to-face with the 30-something Leninist who rarely leaves his isolated nation.The men represent vastly different countries. Trump commands the world's largest economy and most powerful military, while Kim leads an impoverished state that has diverted scarce resources to build a nuclear arsenal to shield the regime from a U.S. attack.Also read: Would invite Kim toUS if summit goes well. If not, will walk away: TrumpTheir diplomatic approaches are worlds apart, too. Trump is known for his volatility and sudden policy reversals, while Kim has methodically played his rivals against each other. So the