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 = 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 = 40007;

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 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] = 1; f[u] = 0;
    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 = 0,get_dis(u,0);
    sort(cdis+1,cdis+cnt+1);
    int l = 1,r = cnt,sum = 0;
    while(l < r) {
        if(cdis[l] + cdis[r] <= k) sum += r-l,++l;
        else --r; 
    }
    return sum;
}

void new_tree(int u) {
    vis[u] = 1; dis[u] = 0;//
    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 = 0,Tsize = size[v];
        get_rt(v,0); new_tree(rt);
    }
} 


void init() {
    memset(head,0,sizeof(head));
    memset(vis,0,sizeof(vis));
    Tsize = n,rt = 0,f[0] = n+1; ans = 0;
}

int main() {
    freopen("input.txt","r",stdin);
    in(n);
    init(); int x,y,w;
    up(i,1,n-1) in(x),in(y),in(w),add(x,y,w),add(y,x,w);in(k);
    get_rt(1,0); new_tree(rt);
    out(ans); putchar('
');
}