Codeforces 280C Game on Tree 期望

#include<bits/stdc++.h>
#define LL long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ull unsigned long long
using namespace std;

const int N = 1e5 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-8;

int n;
vector<int> G[N];
double ans;

void dfs(int u, int fa, int depth) {
    ans += 1.0 / depth;
    for(int v : G[u])
        if(v != fa) dfs(v, u, depth + 1);
}

int main() {
    scanf("%d", &n);
    for(int i = 1; i < n; i++) {
        int u, v; scanf("%d%d", &u, &v);
        G[u].push_back(v);
        G[v].push_back(u);
    }
    dfs(1, 0, 1);
    printf("%.12f\n", ans);
    return 0;
}

/*
*/