【DP】 codeforces 500D New Year Santa Network

#include <iostream>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <bitset>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <climits>
#include <cstdlib>
#include <cmath>
#include <time.h>
#define maxn 200005
#define maxm 2000005
#define eps 1e-10
#define mod 1000000007
#define INF 0x3f3f3f3f
#define PI (acos(-1.0))
#define lowbit(x) (x&(-x))
#define mp make_pair
#define ls o<<1
#define rs o<<1 | 1
#define lson o<<1, L, mid
#define rson o<<1 | 1, mid+1, R
#define pii pair<int, int>
//#pragma comment(linker, "/STACK:16777216")
typedef long long LL;
typedef unsigned long long ULL;
//typedef int LL;
using namespace std;
LL qpow(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base;base=base*base;b/=2;}return res;}
LL powmod(LL a, LL b){LL res=1,base=a;while(b){if(b%2)res=res*base%mod;base=base*base%mod;b/=2;}return res;}
// head

struct Edge
{
int v, w, id;
Edge *next;
}*H[maxn], E[maxm], *edges;

int size[maxn];
int vis[maxn];
LL aa[maxn];
LL bb[maxn];
LL w[maxn];
int n, m;

void addedges(int u, int v, int id)
{
edges->v = v;
edges->id = id;
edges->next = H[u];
H[u] = edges++;
}

void dfs(int u)
{
size[u] = 1;
for(Edge *e = H[u]; e; e = e->next) {
if(!vis[e->v]) {
vis[e->v] = 1;
dfs(e->v);
aa[e->id] = size[e->v];
bb[e->id] = n - aa[e->id];
size[u] += size[e->v];
}
}
}

void init(void)
{
edges = E;
memset(H, 0, sizeof H);
}

void read(void)
{
int a, b, c;
scanf("%d", &n);
for(int i = 1; i < n; i++) {
scanf("%d%d%d", &a, &b, &c);
w[i] = c;
addedges(a, b, i);
addedges(b, a, i);
}
scanf("%d", &m);
}

void work(void)
{
vis[1] = 1;
dfs(1);
int k, t;
double ans = 0;
for(int i = 1; i < n; i++) ans += (aa[i] - 1) * aa[i] * bb[i] * w[i] + aa[i] * bb[i] * (bb[i] - 1) * w[i];
while(m--) {
scanf("%d%d", &k, &t);
ans -= (aa[k] - 1) * aa[k] * bb[k] * (w[k] - t) + aa[k] * bb[k] * (bb[k] - 1) * (w[k] - t);
w[k] = t;
printf("%.9f\n", ans * 6 / n / (n - 1) / (n - 2));
}
}

int main(void)
{
init();
read();
work();

return 0;
}