Project Euler - Problem 40

euler.java

public class Champernowne {
	
	private static String num = null;
	
	public static void main(String[] args) {
		StringBuilder sb = new StringBuilder();
		
		for (int i = 0; i < 1_000_000; i++) {
			sb.append(String.valueOf(i));
			if (i == 0) sb.append('.');
		}
		
		num = sb.toString();
		
		int n = d(1);
		for (int i = 1; i < 7; i++) {
			int pow = (int) Math.pow(10, i);
			n *= d(pow);
		}
		System.out.println(n);
	}
	
	public static int d(int n) {
		if (num == null) throw new Error("num is null");
		return num.charAt(n + 1) -  48;
	}
}