ll det(ll x){
	ll res = 1, w = 1;
	for(int i = 2; i <= x; ++ i){
		for(int j = i + 1; j <= x; ++ j){
			ll val = a[j][i] * qp(a[i][i], P-2) % P;
			for(int k = i; k <= x; ++ k){
				a[j][k] = (a[j][k] - a[i][k] * val % P + P) % P;
			}
		}
		res = res * a[i][i] % P;
	}
	return res * w % P;
}

过了题，但是不用考虑什么 $a[i][i]=0$ 的情况吗