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P2052 [NOI2011]道路修建

题目:

P2052 [NOI2011]道路修建

解析:

维护一下每个子树的\(size\),这条边的贡献就是\(((n-size[v])-size[v])\times w=(n-2\times size[v])\times w\)\(v\)是这条边所到达的点,\(w\)是边权

代码:

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int N = 2e6 + 10;

int n, m, num, ans;
int head[N], size[N];

struct node {
    int v, nx, w;
} e[N];

inline void add(int u, int v, int w) {
    e[++num] = (node) {v, head[u], w}, head[u] = num;
}

void dfs(int u, int fa) {
    size[u] = 1;
    for (int i = head[u]; ~i; i = e[i].nx) {
        int v = e[i].v;
        if (v == fa) continue;
        dfs(v, u);
        ans += abs(n - 2 * size) * e[i].w;
        size[u] += size[v];
    }
}

signed main() {
    ios::sync_with_stdio(false);
    memset(head, -1, sizeof head);
    cin >> n;
    for (int i = 1, x, y, z; i < n; ++i)
        cin >> x >> y >> z, add(x, y, z), add(y, x, z);
    dfs(1, 0);
    cout << ans;
}
posted @ 2019-10-29 16:31  Chrety  阅读(190)  评论(0编辑  收藏  举报