讲解：
https://rpdreamer.blog.luogu.org/ci-fen-and-shu-shang-ci-fen
1 #include <bits/stdc++.h> 2 #define read read() 3 #define up(i,l,r) for(register int i = (l);i <= (r);i++) 4 #define down(i,l,r) for(register int i = (l);i >= (r);i--) 5 #define traversal_vedge(i) for(register int i = head[u]; i ;i = e[i].nxt) 6 #define ll long long 7 using namespace std; 8 int read 9 { 10 int x = 0, f = 1; char ch = getchar(); 11 while(ch < 48 || ch > 57) {if(ch == '-')f = -1; ch = getchar();} 12 while(ch >=48 && ch <=57) {x = 10 * x + ch - 48;ch = getchar();} 13 return x * f; 14 } 15 //-------------------------------------------------------------------- 16 const int N = 50005; 17 int n,k; 18 struct edge{ 19 int v,nxt; 20 }e[N<<1]; int tot,head[N]; 21 void add(int u,int v) {e[++tot] = (edge){v,head[u]};head[u] = tot;} 22 23 int dep[N],size[N],fa[N],top[N]; 24 int cnt[N],maxp = 0; 25 26 void dfs(int u) 27 { 28 dep[u] = dep[fa[u]]+1; 29 size[u] = 1; 30 top[u] = u; 31 int hson_id = 0,hson_size = 0; 32 traversal_vedge(i) 33 { 34 int v = e[i].v; 35 if(v == fa[u]) continue; 36 fa[v] = u; 37 dfs(v); 38 size[u] += size[v]; 39 if(size[v] > hson_size) hson_id = v,hson_size = size[v]; 40 } 41 if(hson_id) top[hson_id] = u; 42 } 43 44 int find(int u) 45 { 46 if(top[u] == u) return u; 47 top[u] = find(top[u]); 48 return top[u]; 49 } 50 51 int lca(int x,int y) 52 { 53 if(find(x) != find(y)) 54 { 55 if(dep[top[x]] > dep[top[y]]) return lca(fa[top[x]],y); 56 else return lca(x,fa[top[y]]); 57 } 58 return dep[x] > dep[y] ? y : x; 59 } 60 61 void dfs_tree(int u) 62 { 63 traversal_vedge(i) 64 { 65 int v = e[i].v; 66 if(v == fa[u]) continue; 67 dfs_tree(v); 68 cnt[u] += cnt[v]; 69 } 70 maxp = max(maxp,cnt[u]); 71 } 72 73 void readdata() 74 { 75 n = read; k = read; 76 up(i,1,n-1) 77 { 78 int u = read, v = read; 79 add(u,v); 80 add(v,u); 81 } 82 dfs(1); 83 up(i,1,k) 84 { 85 int u = read, v = read; 86 cnt[u]++; cnt[v]++; 87 int LCA = lca(u,v); 88 cnt[LCA]--; cnt[fa[LCA]]--; 89 } 90 dfs_tree(1); 91 printf("%d ",maxp); 92 } 93 94 int main() 95 { 96 freopen("input.txt","r",stdin); 97 readdata(); 98 return 0; 99 }