Solution to a problem P1407

Built Figure way: even the old man woman relationship, the relationship between man and even now a woman (of course, in turn, can)

The reason it can be considered:

If a man the woman was green, then this woman will find she had gone out of a man, that is, in this case, some kind of "influence" will spread along the old relations man from the woman, and At this point the man will follow the original relationship between this "influence" to pass another woman; if this "influence" returned the man, it shows that men are successful pairing, and this "impact" of transfer the positive side and the same number on the reverse side of the path (ie, the number of broken relationships and the number of newly-established the same), so this marriage is unstable.

So after completing construction diagram can run Tarjan, if a couple in the same SCC, this marriage is unstable.

#include <cstdio>
#include <iostream>
#include <map>
#include <string>
using namespace std;
map<string, int> M;
const int MAXN = 200000;
int cnt;
int head[MAXN], nxt[MAXN], to[MAXN];
int tot;
void ins(int u, int v)
{
    nxt[++cnt] = head[u];
    head[u] = cnt;
    to[cnt] = v;
}
int dfn[MAXN], low[MAXN], clo[MAXN];
int vis[MAXN], sta[MAXN], top;
void tarjan(int u)
{
    sta[++top] = u;
    dfn[u] = low[u] = ++tot;
    vis[u] = 1;
    for (int i = head[u], v = to[i]; i; i = nxt[i], v = to[i])
        if (!dfn[v])
        {
            tarjan(v);
            low[u] = min(low[u], low[v]);
        }
        else if (vis[v])
            low[u] = min(low[u], dfn[v]);
    if (low[u] == dfn[u])
    {
        clo[u] = u;
        vis[u] = 0;
        int y;
        while ((y = sta[top--]) != u)
            clo[y] = u, vis[y] = 0;
    }
}
int main()
{
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
    int n, m;
    int u, v;
    string girl, boy;
    cin >> n;
    for (int i = 1; i <= n; ++i)
    {
        cin >> girl;
        u = M[girl] = ++tot;
        cin >> boy;
        v = M[boy] = ++tot;
        ins(u, v);
    }
    cin >> m;
    tot = 0;
    for (int i = 1; i <= m; ++i)
    {
        cin >> girl;
        cin >> boy;
        v = M[girl];
        u = M[boy];
        ins(u, v);
    }
    for (int i = 1; i <= (n << 1); ++i)
        if (!dfn[i])
            tarjan(i);
    for (int i = 1; i <= n; ++i)
        if (clo[i << 1] == clo[(i << 1) - 1])
            cout << "Unsafe\n";
        else
            cout << "Safe\n";
    return 0;
}