Competitive Programming

/**
 * Author: Simon Lindholm
 * Date: 2016-09-06
 * License: CC0
 * Source: folklore
 * Description: Calculates determinant of a matrix. Destroys the matrix.
 * Status: somewhat tested on https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&category=&problem=625 (ll and long double)
 * Time: $O(N^3)$
 */
#pragma once

double det(vector<vector<double>>& a) {
	int n = sz(a); double res = 1;
	FOR(i,0,n) {
		int b = i;
		FOR(j,i+1,n) if (fabs(a[j][i]) > fabs(a[b][i])) b = j;
		if (i != b) swap(a[i], a[b]), res *= -1;
		res *= a[i][i];
		if (res == 0) return 0;
		FOR(j,i+1,n) {
			double v = a[j][i] / a[i][i];
			if (v != 0) FOR(k,i+1,n) a[j][k] -= v * a[i][k];
		}
	}
	return res;
}

ll det(vector<vector<ll>>& a, ll mod) {
	int n = sz(a); ll ans = 1;
	FOR(i,0,n) {
		FOR(j,i+1,n) {
			while (a[j][i] != 0) { // gcd step
				ll t = a[i][i] / a[j][i];
				if (t) FOR(k,i,n)
					a[i][k] = (a[i][k] - a[j][k] * t) % mod;
				swap(a[i], a[j]);
				ans *= -1;
			}
		}
		ans = ans * a[i][i] % mod;
		if (!ans) return 0;
	}
	return (ans + mod) % mod;
}