Rank of Tetris(topsort)

http://acm.hdu.edu.cn/showproblem.php?pid=1811

#include <stdio.h>
#include <string.h>
#include <queue>
#include <vector>
using namespace std;
const int N=200010;
vector<int>V[N];
queue<int>q;
char oper[N];
int in[N],f[N];
int a[N],b[N];
int n,m,sum;
void init()
{
    sum = n;
    while(!q.empty()) q.pop();
    for (int i = 0; i <= n; i++)
    {
        f[i] = i;
        in[i] = 0;
        V[i].clear();
    }
}
int Find(int x)
{
    if(x!=f[x]) return f[x]=Find(f[x]);
    return x;
}
int Merge(int x,int y)
{
    x = Find(x);
    y = Find(y);
    if(x!=y)
    {
        f[y] = x;
        return 1;
    }
    return 0;
}
void toposort()
{
    int flag = 0;
    for (int i = 0; i < n; i++)
    {
        if(in[i]==0&&Find(i)==i)
            q.push(i);
    }
    while(!q.empty())
    {
        if (q.size()>1)flag = 1;
        int u = q.front();
        q.pop();
        sum--;
        for (int i = 0 ; i < V[u].size(); i++)
        {
            if(--in[V[u][i]]==0)
                q.push(V[u][i]);
        }
    }
    if (sum > 0) printf("CONFLICT
");
    else if (flag) printf("UNCERTAIN
");
    else printf("OK
");
}
int main()
{
    while(~scanf("%d%d",&n,&m))
    {
        init();
        int u,v;
        for (int i = 0; i < m; i++)
        {
            scanf("%d %c %d",&a[i],&oper[i],&b[i]);
            if(oper[i]=='=')
            {
                if(Merge(a[i],b[i]))
                    sum--;
            }
        }
        for (int i = 0; i < m; i++)
        {
            if(oper[i]=='=') continue;
            u = Find(a[i]);
            v = Find(b[i]);
            if(oper[i]=='>')
            {
                V[u].push_back(v);
                in[v]++;
            }
            if(oper[i]=='<')
            {
                V[v].push_back(u);
                in[u]++;
            }
        }
        toposort();
    }
    return 0;
}