15. Grid Routes

Starting in the top left corner of a 2×2 grid, there are 6 routes (without backtracking) to the bottom right corner.

How many routes are there through a 20×20 grid?

import java.math.BigInteger;
public class Problem15 {
    public void routes() {
        int n=20;
        BigInteger n2f = factorial(2*n);
        BigInteger n1f = factorial(n);
        BigInteger line  = n2f.divide((n1f.multiply(n1f)));

    public BigInteger factorial(int n) {
        BigInteger ret = BigInteger.ONE;
        for (int i = 1; i <= n; ++i) ret = ret.multiply(BigInteger.valueOf(i));
        return ret;

    public static void main(String[] args) {
        Problem15 p = new Problem15();

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