关于单位矩阵

matrix () {
        memset(a,0x3f,sizeof a);
    }
    void build(){
        for(re i=1;i<=n;++i) a[i][i]=0;
    }
    friend matrix operator * (const matrix &x,const matrix &y){
        matrix z;
        for(re i=1;i<=n;++i)
            for(re j=1;j<=n;++j)
                for(re k=1;k<=m;++k)
                    z.a[i][j]=min(z.a[i][j],x.a[i][k]+y.a[k][j]);
        return z;
    };
    matrix qpow(matrix k,int n){
        matrix s;s.build();
        for(;n;n>>=1,k=k*k)if(n&1)s=s*k;
        return s;
    }
};