#10131 「一本通 4.4 例 2」暗的连锁

https://loj.ac/problem/10131

树上边差分，需要注意的是cnt[1] == 0 不能算的问题（他实际并未有连0号点）

 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 using namespace std;
 6 int read{
 7     int x = 0, f = 1; char ch = getchar();
 8     while(ch < 48 || ch > 57) {if(ch == '-') f = -1; ch = getchar();}
 9     while(ch >= 48&& ch <=57) {x = 10 * x + ch - 48; ch = getchar();}
10     return x * f;
11 }
12 //-----------------------------------------------------------------
13 const int N = 1e5+7;
14 int n,m;
15 
16 struct edge{
17     int v,nxt;
18 }e[N<<1];int tot,head[N];
19 
20 void add(int u,int v){e[++tot] = (edge){v,head[u]}; head[u] = tot; }
21 //-----------------------------------------------------------------
22 
23 int fa[N],size[N],top[N],dep[N]; int cnt[N],ans;
24 
25 void dfs(int u){
26     size[u] = 1;
27     top[u] = u;
28     dep[u] = dep[fa[u]] + 1;
29     int h_size = 0, h_id = 0;
30     for(int i = head[u]; i; i = e[i].nxt){
31         int v = e[i].v;
32         if(v == fa[u]) continue;
33         fa[v] = u;
34         dfs(v);
35         size[u] += size[v];
36         if(size[v] > h_size) h_size = size[v], h_id = v;
37     }
38     if(h_id) top[h_id] = u;
39 }
40 
41 int find(int u){
42     if(top[u] == u) return u;
43     top[u] = find(top[u]);
44     return top[u];
45 }
46 
47 int lca(int u,int v){
48     if(find(u) != find(v)){
49         if(dep[top[u]] > dep[top[v]]) return lca(fa[top[u]],v);
50         else return lca(u,fa[top[v]]);
51     }
52     return dep[u] > dep[v] ? v : u;
53 }
54 //-----------------------------------------------------------------
55 
56 void dfs_edge(int u,int f){
57     
58     for(int i = head[u]; i; i = e[i].nxt){
59         int v = e[i].v;
60         if(v == f) continue;
61         dfs_edge(v,u);
62         cnt[u] += cnt[v];
63     }
64     //if(u == 1) return;//!!
65     if(cnt[u] == 0) ans += m;
66     else if(cnt[u] == 1) ++ans;
67 }
68 
69 void work(){
70     dfs(1);
71     cnt[1] = -1;
72     up(i,1,m){
73         int u = read,v = read;
74         ++cnt[u]; ++cnt[v]; cnt[(lca(u,v))] -= 2;
75     }
76     dfs_edge(1,0);
77     printf("%d\n",ans);
78 }
79 
80 void readdata(){
81     n = read; m = read;
82     int u,v;
83     up(i,1,n-1){
84         u = read; v = read;
85         add(u,v);
86         add(v,u);
87     }
88 }
89 
90 int main(){
91     freopen("input.txt","r",stdin);
92     readdata();
93     work();
94     return 0;
95 }