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 = 0; T f = 1; char ch = getchar();
    while(!isdigit(ch)) {if(ch == '-') f = -1; ch = getchar();}
    while( isdigit(ch)) {x = 10 * x + ch - 48; ch = getchar();}
    x *= f;
}
template <typename T> void out(T x) {
    if(x < 0) x = -x , putchar('-');
    if(x > 9) out(x/10);
    putchar(x%10 + 48);
}
//---------------------------------------------------------
const int N = 2e5+7,inf = 1e9+7;
int n,k;

struct edge {
    int v,w,nxt;
}e[N<<1]; 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[1000007];

bool vis[N];
int ans = inf;

void get_rt(int u,int fa) {
    size[u] = 1; f[u] = 0;
    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] + 1;
        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] = 1; t[0] = 0; //dep[u] = 0;
    for(int i = head[u]; i; i = e[i].nxt) {
        int v = e[i].v; if(vis[v]) continue;
        dep[v] = 1,dis[v] = e[i].w; get_ans(v,0); update_t(v,0,1);
    }
    for(int i = head[u]; i; i = e[i].nxt) {
        int v = e[i].v; if(vis[v]) continue;
        update_t(v,0,0);
    }
    for(int i = head[u]; i ; i = e[i].nxt) {
        int v = e[i].v; if(vis[v]) continue;
        Tsize = size[v],rt = 0; get_rt(v,0); solve(rt);
        //rt - > v
    }
}

int main() {
    in(n); in(k); int x,y,w;
    up(i,1,n-1) in(x),in(y),in(w),add(x+1,y+1,w),add(y+1,x+1,w);
    up(i,0,k) t[i] = inf;
    Tsize = n,rt = 0,f[0] = n+1; get_rt(1,0); solve(rt);
    if(ans != inf) out(ans); else out(-1);
    return 0;
}