树上差分——点差分裸题 P3128 [USACO15DEC]最大流Max Flow

讲解：

https://rpdreamer.blog.luogu.org/ci-fen-and-shu-shang-ci-fen

 #include <bits/stdc++.h>
 #define read read()
 #define up(i,l,r) for(register int i = (l);i <= (r);i++)
 #define down(i,l,r) for(register int i = (l);i >= (r);i--)
 #define traversal_vedge(i) for(register int i = head[u]; i ;i = e[i].nxt)
 #define ll long long
 using namespace std;
 int read
 {
     int x = , f = ; char ch = getchar();
     while(ch <  || ch > ) {if(ch == '-')f = -; ch = getchar();}
     while(ch >= && ch <=) {x =  * x + ch - ;ch = getchar();}
     return x * f;
 }
 //--------------------------------------------------------------------
 const int N = ;
 int n,k;
 struct edge{
     int v,nxt;
 }e[N<<]; int tot,head[N];
 void add(int u,int v) {e[++tot] = (edge){v,head[u]};head[u] = tot;}

 int dep[N],size[N],fa[N],top[N];
 int cnt[N],maxp = ;

 void dfs(int u)
 {
     dep[u] = dep[fa[u]]+;
     size[u] = ;
     top[u] = u;
     int hson_id = ,hson_size = ;
     traversal_vedge(i)
     {
         int v = e[i].v;
         if(v == fa[u]) continue;
         fa[v] = u;
         dfs(v);
         size[u] += size[v];
         if(size[v] > hson_size) hson_id = v,hson_size = size[v];
     }
     if(hson_id) top[hson_id] = u;
 }

 int find(int u)
 {
     if(top[u] == u) return u;
     top[u] = find(top[u]);
     return top[u];
 }

 int lca(int x,int y)
 {
     if(find(x) != find(y))
     {
         if(dep[top[x]] > dep[top[y]]) return lca(fa[top[x]],y);
         else return lca(x,fa[top[y]]);
     }
     return dep[x] > dep[y] ? y : x;
 }

 void dfs_tree(int u)
 {
     traversal_vedge(i)
     {
         int v = e[i].v;
         if(v == fa[u]) continue;
         dfs_tree(v);
         cnt[u] += cnt[v];
     }
     maxp = max(maxp,cnt[u]);
 }

 void readdata()
 {
     n = read; k = read;
     up(i,,n-)
     {
         int u = read, v = read;
         add(u,v);
         add(v,u);
     }
     dfs();
     up(i,,k)
     {
         int u = read, v = read;
         cnt[u]++; cnt[v]++;
         int LCA = lca(u,v);
         cnt[LCA]--; cnt[fa[LCA]]--;
     }
     dfs_tree();
     printf("%d\n",maxp);
 }

 int main()
 {
     freopen("input.txt","r",stdin);
     readdata();
     return ;
 }