SPOJ Find The Determinant III 计算行列式

https://www.spoj.com/problems/DETER3/en/

题意：计算行列式

#include <iostream>
#include<cstdio>
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll a[205][205];
ll mode;
int n;
ll det(){
	int i,j,k;
	ll  ans=1;
	for(i=0;i<n;i++){
		//对角线上为0需要找到这个位置不为0的和他交换 
		if(!a[i][i]){
			bool flag=false ;
			for(j=i+1;j<n;j++){
				if(a[j][i]){
					flag=true;
					for(k=0;k<n;k++)swap(a[i][k],a[j][k]);
					ans=-ans;
					break;
				}
			}
			if(!flag)return 0;//不满秩
		}
		for(j=i+1;j<n;j++){
			while(a[j][i]){//消去下三角种不为0的位置
				ll tmp=a[i][i]/a[j][i];
				for(k=i;k<n;k++){
					a[i][k]=(a[i][k]-tmp*a[j][k])%mode;
					swap(a[i][k],a[j][k]);
				}
				ans=-ans;
			}
		}
		ans=(ans*a[i][i])%mode;
	}
	return (ans%mode+mode)%mode;
}
int main()
{
	int i,j;
	while(cin>>n>>mode){
		for(i=0;i<n;i++){
			for(j=0;j<n;j++){
				cin>>a[i][j];
			}
		}
		printf("%lld\n",det());
	}
  //  cout << "Hello world!" << endl;
    return 0;
}