比较优的一个解

public class Hello {
	public static void main(String[] args) {
		double pi = 0;
		int sign = 1;
		for (int i = 0; i < 10000000; i++) {
			pi += sign * (double) 1 / (2 * i + 1);
			sign *= -1;
		}
		System.out.println("pi=" + 4 * pi);
	}
}