POJ3417（树上差分）

会卡vector。

 const int maxn = 1e5 + ;
 int n, m, Ans;
 int head[maxn], nxt[maxn * ], to[maxn * ], tot;
 int vis[maxn], f[maxn];
 int d[maxn], sum[maxn];
 vector<int> q[maxn];

 inline void Add(int u, int v) {
     to[++tot] = v, nxt[tot] = head[u], head[u] = tot;
 }

 inline int getf(int v) { return v == f[v] ? v : f[v] = getf(f[v]); }

 inline void Tarjan(int cur) {
     vis[cur] = ;

     for (int j = head[cur]; j; j = nxt[j]) {
         int i = to[j];
         if (vis[i])    continue;
         Tarjan(i);
         f[i] = cur;
         sum[cur] += sum[i];
     }

     vis[cur] = ;

     for (int j = ; j < q[cur].size(); ++j) {
         int i = q[cur][j];
         if (vis[i] == ) {
             int lca = getf(i);
             d[lca] -= ;
         }
     }

     sum[cur] += d[cur];
     if (sum[cur] == )    Ans += m;
     else if (sum[cur] == )    Ans++;
 }

 int main() {
     read(n), read(m);

     rep(i, , n)
         f[i] = i;

     rep(i, , n - ) {
         int u, v;
         read(u), read(v);
         Add(u, v), Add(v, u);
     }

     rep(i, , m) {
         int u, v;
         read(u), read(v);
         d[u]++, d[v]++;

         q[u].push_back(v);
         if (u != v)    q[v].push_back(u);
     }

     Tarjan();

     writeln(Ans - m);//1上面没有边，多算了m

     return ;
 }