Luogu3119 草鉴定-Tarjan+Topsort

Solution

简单的$Tarjan$题。

有大佬现成博客 就不写了 → 传送门

Code

 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 #include<queue>
 #define rd read()
 using namespace std;

 const int N = 1e5 + ;
 const int inf = ~0U >> ;

 int n, m;
 int head[N], tot, Head[N], Tot;
 int low[N], dfn[N], cnt;
 int st[N], tp, col, c[N], val[N], subsz[N], upsz[N], deg[N], bvis[N];
 bool vis[N];

 struct edge {
     int nxt, fr, to;
 }e[N], E[N];

 queue<int> q;

 int read() {
     int X = , p = ; char c = getchar();
     for (; c > '' || c < ''; c = getchar())
         if (c == '-') p = -;
     for (; c >= '' && c <= ''; c = getchar())
         X = X *  + c - '';
     return X * p;
 }

 void add(int u, int v) {
     e[++tot].to = v;
     e[tot].fr = u;
     e[tot].nxt = head[u];
     head[u] = tot;
 }

 void Add(int u, int v) {
     E[++Tot].to = v;
     E[Tot].fr = u;
     E[Tot].nxt = Head[u];
     Head[u] = Tot;
 }

 void cmin(int &A, int B) {
     if (A > B) A = B;
 }

 void cmax(int &A, int B) {
     if (A < B) A = B;
 }

 void tarjan(int u) {
     dfn[u] = low[u] = ++cnt;
     vis[u] = ; st[++tp] = u;
     for (int i = head[u]; i; i = e[i].nxt) {
         int nt = e[i].to;
         if (!dfn[nt]) {
             tarjan(nt);
             cmin(low[u], low[nt]);
         }
         else if (vis[nt]) cmin(low[u], dfn[nt]);
     }
     if (low[u] == dfn[u]) {
         col++;
         for (; tp;) {
             int z = st[tp--];
             c[z] = col;
             vis[z] = ;
             val[col]++;
             if (z == u) break;
         }
     }
 }

 void dfs(int u) {
     vis[u] = ;
     for (int i = Head[u]; i; i = E[i].nxt) {
         int nt = E[i].to;
         dfs(nt);
     }
 }

 int dfs2(int u) {
     if (bvis[u] == ) return ;
     if (bvis[u] == -) return ;
     if (u == c[]) return ;
     bool flag = false;
     for (int i = Head[u]; i; i = E[i].nxt) {
         int nt = E[i].to;
         if (dfs2(nt)) {
             flag = true;
             cmax(upsz[u], upsz[nt] + val[u]);
         }
     }
     bvis[u] = flag ?  : -;
     return flag;
 }

 void Topsort() {
     subsz[c[]] = val[c[]];
     for (int i = ; i <= col; ++i)
         if (!deg[i]) q.push(i);
     for (int u; !q.empty(); ) {
         u = q.front(); q.pop();
         for (int i = Head[u]; i; i = E[i].nxt) {
             int nt = E[i].to;
             if (subsz[u] != -inf)
                 cmax(subsz[nt], subsz[u] + val[nt]);
             deg[nt]--;
             if (!deg[nt]) q.push(nt);
         }
     }
 }

 int main()
 {
     n = rd; m = rd;
     for (int i = ; i <= m; ++i) {
         int u = rd, v = rd;
         add(u, v);
     }
     for (int i = ; i <= n; ++i)
         if (!dfn[i]) tarjan(i);
     for (int i = ; i <= m; ++i) {
         int u = e[i].fr, v = e[i].to;
         u = c[u], v = c[v];
         if (u == v) continue;
         Add(u, v);
         deg[v]++;
     }
     memset(vis, , sizeof(vis));
     for (int i = ; i <= col; ++i) subsz[i] = -inf;
     dfs(c[]);
     Topsort();
     bvis[c[]] = ;
     upsz[c[]] = val[c[]];
     for (int i = ; i <= col; ++i)
         if (!vis[i]) dfs2(i);
     int ans = val[c[]];
     for (int i = ; i <= m; ++i) {
         int u = e[i].fr, v = e[i].to;
         u = c[u]; v = c[v];
         if (u == v) continue;
         if (!vis[v] || !(bvis[u] == )) continue;
         cmax(ans, subsz[v] + upsz[u] - val[c[]]);
     }
     printf("%d\n", ans);
 }