題目鏈接
$N$個牛，$M$個關系 $(A，B)A$認為$B$是受歡迎的。求受所有牛歡迎的牛的數量。

思路

同一個強聯通分量里面的牛是相互受歡迎的，我們將所有的聯通分量求出來之后，那些出度為零的聯通分量就是答案，前提保證這樣的聯通分量只有一個。

#include <map> #include <set> #include <cmath> #include <queue> #include <vector> #include <stdio.h> #include <iostream> #include <numeric> #include <algorithm> #include <cstring> #include <time.h> #define LL long long #define P pair<int, int> #define lowbit(x) (x & -x) #define mem(a, b) memset(a, b, sizeof(a)) #define mid ((l + r) >> 1) #define lc rt<<1 #define rc rt<<1|1 #define endl '\n' const int maxn = 1e4 + 5; const int inf = 0x3f3f3f3f; const int mod = 1e9 + 7; using namespace std; vector<int> g[maxn]; int Stack[maxn], low[maxn], dfn[maxn], inStack[maxn], belong[maxn]; int vis[maxn], color[maxn]; int now, len, cnt; void init() {now = len = cnt = 0;mem(inStack, 0);mem(belong, 0);mem(dfn, 0);mem(low, 0); } void tarjan(int x) {low[x] = dfn[x] = ++now;Stack[++len] = x;inStack[x] = 1;int tmp = g[x].size();for (int i = 0; i < tmp; ++i) {int y = g[x][i];if (!dfn[y]) tarjan(y), low[x] = min(low[x], low[y]);else if (inStack[y]) low[x] = min(low[x], low[y]);}if (dfn[x] == low[x]) {++cnt;int top;while (Stack[len] != x) {top = Stack[len--];belong[top] = cnt;inStack[top] = 0;}top = Stack[len--];belong[top] = cnt;inStack[top] = 0;} } int main() {ios::sync_with_stdio(false);cin.tie(0); cout.tie(0);int n, m;while (cin >> n >> m) {for (int i = 1; i <= n; ++i) {g[i].clear();}for (int i = 0, u,v; i < m; ++i) {cin >> u >> v;g[u].push_back(v);}init();for (int i = 1; i <= n; ++i) {if (dfn[i]) continue;tarjan(i);}mem(vis, 0);for (int i = 1; i <= n; ++i) {for (int j = 0; j < (int)g[i].size(); ++j) {int v = g[i][j];int l = belong[i];int r = belong[v];if (l == r) continue;vis[l] = 1;}}int sum = 0, ans = 0, flag;for (int i = 1; i <= n; ++i) {if (vis[belong[i]] == 0) {vis[belong[i]] = 1;sum++;flag = belong[i];}}for (int i = 1; i <= n; ++i) {if (belong[i] == flag) ans++;}if (sum != 1) ans = 0;cout << ans << endl;}return 0; }/* 3 3 1 2 2 3 3 13 3 1 2 2 1 2 35 4 1 4 2 4 3 4 5 45 5 1 2 2 3 3 1 1 4 4 55 6 1 2 2 3 3 1 1 4 4 5 5 32 2 1 2 2 13 2 1 2 2 16 6 1 2 2 3 3 1 1 4 4 5 5 35 6 1 2 2 3 3 1 1 4 4 5 5 45 7 4 1 1 2 2 3 3 1 1 4 4 5 5 45 6 1 2 2 3 3 1 1 4 4 5 5 17 9 1 2 2 3 3 1 4 5 5 6 6 4 4 7 7 1 1 76 6 1 2 2 3 3 1 4 5 5 6 6 44 4 1 2 2 3 3 1 1 44 4 1 2 2 3 3 1 4 15 6 1 2 2 3 3 1 5 1 5 4 3 47 9 1 2 2 3 3 1 5 1 5 4 3 4 4 7 7 6 6 43 1 1 1 5 2 0 0 2 5 5 4 0 1 3 1 3 */

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