BZOJ 4773: 负环 倍增Floyd

Code： 

#include <cstdio> 
#include <cstring> 
#include <algorithm>
#define N 303 
#define inf 1000000000 
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std; 
namespace IO
{
    char *p1, *p2, buf[100000];
    #define nc() (p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 100000, stdin), p1 == p2) ? EOF : *p1 ++ )
    int rd() { int x = 0, f = 1;char c = nc();while (c < 48) {if (c == '-')f = -1;c = nc();}while (c > 47) {x = (((x << 2) + x) << 1) + (c ^ 48), c = nc();}return x * f;}
}; 
int n,m,dis[N][N][10],tmp[2][N][N];                 
inline void getmin(int &a,int b) { if(b<a)a=b; }      
inline int check() 
{
    int i; 
    for(i=1;i<=n;++i) if(tmp[1][i][i]<0) return 1; 
    return 0;   
}
int main() 
{
    int i,j,k,l,ans=inf;      
    // setIO("input"); 
    n=IO::rd(),m=IO::rd(); 
    for(i=1;i<=n;++i) 
    {
        for(j=1;j<=n;++j) 
            for(k=0;k<10;++k) dis[i][j][k]=inf;     
    }
    for(i=1;i<=n;++i) for(k=0;k<10;++k) dis[i][i][k]=0;     
    for(i=1;i<=m;++i) 
    {
        int a=IO::rd(),b=IO::rd(),c=IO::rd(); 
        getmin(dis[a][b][0], c);    
    }   
    for(l=1;l<10;++l)  
    {
        for(k=1;k<=n;++k) 
            for(i=1;i<=n;++i) 
            {
                if(dis[i][k][l-1]==inf) continue;   
                for(j=1;j<=n;++j) 
                {
                    if(dis[i][k][l-1]<inf && dis[k][j][l-1]<inf)    
                        getmin(dis[i][j][l],dis[i][k][l-1]+dis[k][j][l-1]);   
                } 
            }
    }               
    for(i=1;i<=n;++i) for(j=1;j<=n;++j) tmp[0][i][j]=inf; 
    for(i=1;i<=n;++i) tmp[0][i][i]=0;       
    int now=0; 
    for(l=9;l>=0;--l) 
    {   
        for(i=1;i<=n;++i) for(j=1;j<=n;++j) tmp[1][i][j]=tmp[0][i][j];     
        for(k=1;k<=n;++k)   
        for(i=1;i<=n;++i) 
        { 
            if(tmp[0][i][k]==inf) continue;     
            for(j=1;j<=n;++j)        
                    getmin(tmp[1][i][j], tmp[0][i][k]+dis[k][j][l]);   
        }          
        if(check()) getmin(ans,now|(1<<l));   
        else 
        {
            now|=(1<<l);    
            for(i=1;i<=n;++i) for(j=1;j<=n;++j) tmp[0][i][j]=tmp[1][i][j];   
        }
    }
    printf("%d\n",ans==inf?0:ans);    
    return 0;   
}