P4149 [IOI2011]Race

　　对于这道题,明显是点分治,权值等于k,可以用桶统计树上路径(但注意要清空);

对于每颗子树,先与之前的子树拼k,再更新桶,维护t["len"]最小边数;

 #include <bits/stdc++.h>
 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 = 2e5+,inf = 1e9+;
 int n,k;

 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 Tsize,rt;
 int f[N],size[N];
 int dis[N],dep[N];

 int t[];

 bool vis[N];
 int ans = inf;

 void get_rt(int u,int fa) {
     size[u] = ; f[u] = ;
     for(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];
         //if(f[u] < size[v]) f[u] = size[v];
         f[u] = max(f[u],size[v]);
     }
     f[u] = max(f[u],Tsize-size[u]);
     if(f[u] < f[rt]) rt = u;
 }

 void get_ans(int u,int fa) {
     //if(dis[u] > k) return;//>
     if(dis[u] <= k)
     ans = min(ans,t[k-dis[u]]+dep[u]);
     for(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; dep[v] = dep[u] + ;
         get_ans(v,u); //!!!!!未写
     }
 }

 void update_t(int u,int fa,bool flag) {
     //if(dis[u] > k) return;//>
     if(dis[u] <= k) {
         if(flag) t[dis[u]] = min(t[dis[u]],dep[u]);
         else t[dis[u]] = inf;
     }
     for(int i = head[u]; i ;i = e[i].nxt) {
         int v = e[i].v; if(v == fa || vis[v]) continue;
         update_t(v,u,flag);
     }
 }

 void solve(int u) {
     vis[u] = ; t[] = ; //dep[u] = 0;
     for(int i = head[u]; i; i = e[i].nxt) {
         int v = e[i].v; if(vis[v]) continue;
         dep[v] = ,dis[v] = e[i].w; get_ans(v,); update_t(v,,);
     }
     for(int i = head[u]; i; i = e[i].nxt) {
         int v = e[i].v; if(vis[v]) continue;
         update_t(v,,);
     }
     for(int i = head[u]; i ; i = e[i].nxt) {
         int v = e[i].v; if(vis[v]) continue;
         Tsize = size[v],rt = ; get_rt(v,); solve(rt);
         //rt - > v
     }
 }

 int main() {
     in(n); in(k); int x,y,w;
     up(i,,n-) in(x),in(y),in(w),add(x+,y+,w),add(y+,x+,w);
     up(i,,k) t[i] = inf;
     Tsize = n,rt = ,f[] = n+; get_rt(,); solve(rt);
     if(ans != inf) out(ans); else out(-);
     return ;
 }