Sorting It All Out

http://poj.org/problem?id=1094

#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
#include<vector>
#include<queue>
#include<cstdlib>
using namespace std;
const int maxn=100000+5;
vector <int> g[maxn];
int du[maxn],n,m,l[maxn];
bool vis[1000][1000];
int toposort()
{
    memset(du,0,sizeof(du));
    for(int i=0; i<n; i++)
    {
        for(int j=0; j<g[i].size(); j++)
        {
            du[g[i][j]]++;
        }
    }
    int tot=0;
    bool flag=false;
    queue<int>q;
    for(int i=0; i<n; i++)
    {
        if(!du[i]) q.push(i);
    }
    while(!q.empty())
    {
        if(q.size()>1)
        {
            flag=true;
        }
        int x=q.front();
        q.pop();
        l[tot++]=x;
        for(int j=0; j<g[x].size(); j++)
        {
            int t=g[x][j];
            du[t]--;
            if(!du[t]) q.push(t);
        }
    }
    if(tot!=n) return 1;
    else if(flag) {
        return 2;
    }
    return 0;
}
int main()
{
    char c1,c2;
    while(scanf("%d%d",&n,&m))
    {
        getchar();
        for(int i=0;i<maxn;i++)
        {
            g[i].clear();
        }
        bool flag1=false,flag2=false;
        memset(vis,0,sizeof(vis));
        if(n==0&&m==0) break;
        for(int i=1; i<=m; i++)
        {
            scanf("%c<%c",&c1,&c2);
            getchar();
            if(!flag1&&!flag2)
            {
                if(vis[c2-'A'][c1-'A'])
                {
                    flag1=true;
                    printf("Inconsistency found after %d relations.
",i);
                    continue;
                }
                if(!vis[c1-'A'][c2-'A'])
                {
                    g[c1-'A'].push_back(c2-'A');
                    vis[c1-'A'][c2-'A']=1;
                }
                int k=toposort();
                if(k==0)
                {
                    printf("Sorted sequence determined after %d relations: ",i);
                    for(int j=0; j<n; j++) printf("%c",l[j]+'A');
                    printf(".
");
                    flag2=true;
                }
                else if(k==1)
                {
                    flag1=true;
                    printf("Inconsistency found after %d relations.
",i);
                }
            }
        }
        if(!flag1&&!flag2)
        {
            printf("Sorted sequence cannot be determined.
");
        }
    }
    return 0;
}