LightOJ - 1417 Forwarding Emails(强连通+dfs)

来源：互联网发布：怎么设置农村淘宝收货编辑：程序博客网时间：2024/05/29 15:15

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;}

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