HDU 2376 Average distance

HDU_2376

    对于任意一棵子树来讲，以根节点为深度最浅的点的路径一共有两类，一类是以根节点为端点的路径，另一类是过根节点但端点分别在两棵子树中的路径。然后将无根树转化为有根树后dfs时计算出上面两类路径的长度即可。

#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<queue>
#define MAXD 10010
#define MAXM 20010
typedef long long LL;
LL f[MAXD], ANS;
int N, first[MAXD], size[MAXD], e, next[MAXM], v[MAXM], w[MAXM];
void add(int x, int y, int z)
{
    v[e] = y, w[e] = z;
    next[e] = first[x], first[x] = e ++;
}
void init()
{
    int i, x, y, z;
    scanf("%d", &N);
    memset(first, -1, sizeof(first[0]) * N), e = 0;
    for(i = 1; i < N; i ++)
    {
        scanf("%d%d%d", &x, &y, &z);
        add(x, y, z), add(y, x, z);
    }
}
struct St
{
    LL f;
    int size;
    St(){}
    St(LL _f, int _size) : f(_f), size(_size){}
};
void dfs(int cur, int fa)
{
    int i, s = 0;
    LL A = 0;
    size[cur] = 1, f[cur] = 0;
    std::queue <St> q;
    for(i = first[cur]; i != -1; i = next[i])
        if(v[i] != fa)
        {
            dfs(v[i], cur);
            size[cur] += size[v[i]], f[cur] += f[v[i]] + w[i] * size[v[i]];
            q.push(St(f[v[i]] + size[v[i]] * w[i], size[v[i]]));
        }
    ANS += f[cur];
    while(!q.empty())
    {
        St st = q.front();
        q.pop();
        ANS += st.size * A + s * st.f;
        s += st.size, A += st.f;
    }
}
void solve()
{
    ANS = 0;
    dfs(0, -1);
    printf("%.7f\n", (double)ANS * 2 / (N * (N - 1)));
}
int main()
{
    int t;
    scanf("%d", &t);
    while(t --)
    {
        init();
        solve();
    }
    return 0;
}