poj2723 Get Luffy Out

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <cmath>
#include <stack>

using namespace std;

const int maxn = 10010,maxm = 200010;

int n,m,key[maxn],a[maxn],b[maxn],pre[maxn],low[maxn],scc[maxn],dfs_clock,head[maxn],nextt[maxm],to[maxm],tot = 1,top;

stack<int> s;

void add(int x,int y)
{
    to[tot] = y;
    nextt[tot] = head[x];
    head[x] = tot++;
}

void tarjan(int u)
{
    pre[u] = low[u] = ++dfs_clock;
    s.push(u);
    for (int i = head[u];i;i = nextt[i])
    {
        int v = to[i];
        if (!pre[v])
        {
            tarjan(v);
            low[u] = min(low[u],low[v]);
        }
        else
        if (!scc[v])
        low[u] = min(low[u],pre[v]);
    }
    if (low[u] == pre[u])
    {
        top++;
        while (1)
        {
            int t = s.top();
            s.pop();
            scc[t] = top;
            if (t == u)
            break;
        } 
    }
}

bool solve()
{
    for (int i = 0; i < n * 2; i++)
    if (!pre[i])
    tarjan(i);
    for (int i = 0; i < n * 2; i += 2)
    if (scc[i] == scc[i ^ 1])
    return false;
    return true;
}

int main()
{
    while (scanf("%d%d",&n,&m) && (n || m))
    {
        memset(head,0,sizeof(head));
        tot = 1;
        for (int i = 0; i < n; i++)
        {
            int u,v;
            scanf("%d%d",&u,&v);
            key[u] = i * 2;
            key[v] = i * 2 + 1;
        }
        for (int i = 0; i < m; i++)
        scanf("%d%d",&a[i],&b[i]);
        for (int i = 0; i < m; i++)
        {
            if (a[i] == b[i])
            add(key[a[i]] ^ 1,key[a[i]]); //必须选，所以表示不选的点要连向选的点
            else
            {
                add(key[a[i]] ^ 1,key[b[i]]);
                add(key[b[i]] ^ 1,key[a[i]]);
            }
            memset(pre,0,sizeof(pre));
            memset(low,0,sizeof(low));
            memset(scc,0,sizeof(scc));
            top = 0;
            dfs_clock = 0;
            if (!solve())
            {
                printf("%d\n",i);
                break;
            }
            if (i == m - 1)
            printf("%d\n",m);
        }
    }


    return 0;
}