P4178 Tree

最简单的点分治

淀粉质的思想：

“分而治之”，缩小问题规模，合并求解；

 #include<cstdio>
 #include<cmath>
 #include<cstring>
 #include<algorithm>
 #include<iostream>
 using namespace std;
 #define up(i,l,r) for(register int i = (l); i <= (r); ++i)
 #define dn(i,l,r) for(register int i = (l); i >= (r); --i)
 #define ll long long
 #define re register
 using namespace std;

 template <typename T> void in(T &x) {
     x = ; T f = ; char ch = getchar();
     while(!isdigit(ch)) {if(ch == '-') f = -; ch = getchar();}
     while( isdigit(ch)) {x =  * x + ch - ; ch = getchar();}
     x *= f;
 }

 template <typename T> void out(T x) {
     if(x < ) x = -x , putchar('-');
     if(x > ) out(x/);
     putchar(x% + );
 }
 //---------------------------------------------------------

 const int N = ;

 struct edge {
     int v,w,nxt;
 } e[N<<];int tot,head[N];

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

 int n,k,ans;
 int size[N];
 int Tsize,cnt,rt;
 int cdis[N],dis[N];
 bool vis[N];

 int f[N];
 void get_rt(int u,int fa) {
     size[u] = ; f[u] = ;
     for(re int i = head[u]; i; i = e[i].nxt) {
         int v = e[i].v; if(v == fa || vis[v]) continue;
         get_rt(v,u);
         size[u] += size[v];
         f[u] = max(size[v],f[u]);
     }
     f[u] = max(f[u],Tsize-size[u]);
     if(f[u] < f[rt]) rt = u;
 }

 void get_dis(int u,int fa) {
     cdis[++cnt] = dis[u];
     for(re int i = head[u]; i; i = e[i].nxt) {
         int v = e[i].v; if(v == fa || vis[v]) continue;
         dis[v] = dis[u] + e[i].w; get_dis(v,u);
     }
 }

 int calc(int u) {
     cnt = ,get_dis(u,);
     sort(cdis+,cdis+cnt+);
     int l = ,r = cnt,sum = ;
     while(l < r) {
         if(cdis[l] + cdis[r] <= k) sum += r-l,++l;
         else --r;
     }
     return sum;
 }

 void new_tree(int u) {
     vis[u] = ; dis[u] = ;//
     ans += calc(u);
     for(re int i = head[u]; i; i = e[i].nxt) {
         int v = e[i].v; if(vis[v]) continue;
         dis[v] = e[i].w;//
         ans -= calc(v);
         rt = ,Tsize = size[v];
         get_rt(v,); new_tree(rt);
     }
 } 

 void init() {
     memset(head,,sizeof(head));
     memset(vis,,sizeof(vis));
     Tsize = n,rt = ,f[] = n+; ans = ;
 }

 int main() {
     freopen("input.txt","r",stdin);
     in(n);
     init(); int x,y,w;
     up(i,,n-) in(x),in(y),in(w),add(x,y,w),add(y,x,w);in(k);
     get_rt(,); new_tree(rt);
     out(ans); putchar('\n');
 }