LightOJ - 1417 Forwarding Emails(强连通+dfs)

题目大意：你有一个秘密，但你只能告诉一个。这个人如果有认识的人的话，他就会把你的秘密告诉出去，这样就一传十，十传百了。现在你想要知道，最多能有多少人知道

解题思路：同一个连通分量内的人都是可知的，所以缩点，然后连边，从入度为0的点出发，dfs找出所能到的最远处，在dfs过程中统计一下有多少个人知道

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <map>
using namespace std;
const int N = 50010;

struct Edge{
    int u, v, next;
    Edge() {}
    Edge(int u, int v, int next): u(u), v(v), next(next) {}
}E[N];

int head[N], pre[N], lowlink[N], sccno[N], num[N], Stack[N], in[N];
int dfs_clock, scc_cnt, top, n, m, cas = 1;
bool vis[N];
vector<int> V[N];

void init() {
    scanf("%d", &n);
    memset(head, -1, sizeof(head));

    int u, v;
    for (int i = 0; i  < n; i++) {
        scanf("%d%d", &u, &v);
        E[i] = Edge(u, v, head[u]);
        head[u] = i;
    }
}

void dfs(int u, int fa) {
    pre[u] = lowlink[u] = ++dfs_clock;
    Stack[++top] = u;

    for (int i = head[u]; ~i; i = E[i].next) {
        int v = E[i].v;

        if (!pre[v]) {
            dfs(v, u);
            lowlink[u] = min(lowlink[u], lowlink[v]);
        }
        else if (!sccno[v]) lowlink[u] = min(lowlink[u], pre[v]);
    }

    if (pre[u] == lowlink[u]) {
        scc_cnt++;
        int x = 0;
        num[scc_cnt] = 0;
        while (1) {
            x = Stack[top--];
            sccno[x] = scc_cnt;
            num[scc_cnt]++;
            if (x == u) break;
        }
    }
}

int dfs2(int u){
    int Max = 0;
    for (int i = 0; i < V[u].size(); i++) {
        int v = V[u][i];
        Max = max(Max, dfs2(v));
    }
    return Max + num[u];
}

void solve() {
    memset(pre, 0, sizeof(pre));
    memset(sccno, 0, sizeof(sccno));
    dfs_clock = scc_cnt = 0;

    for (int i = 1; i <= n; i++)
        if (!pre[i]) dfs(i, -1);

    if (scc_cnt == 1) {
        printf("Case %d: %d\n", cas++, 1);
        return ;
    }

    memset(in, 0, sizeof(in));

    for (int i = 1; i <= scc_cnt; i++)
        V[i].clear();

    for (int i = 0; i < n; i++) {
        int u = sccno[E[i].u];
        int v = sccno[E[i].v];

        if (u == v) continue;
        in[v]++;
        V[u].push_back(v);
    }
    int Max = 0, pos;
    for (int i = 1; i <= scc_cnt; i++)
        if (!in[i]) {
            int t = dfs2(i);
            if (t > Max) {
                Max = t;
                pos = i;
            }
        }
    for (int i = 1; i <= n; i++)
        if (sccno[i] == pos) {
            printf("Case %d: %d\n", cas++, i);
            return ;
        }
}

int main() {
    int test;
    scanf("%d", &test);
    while (test--) {
        init();
        solve();
    }
    return 0;
}