蒟蒻不理解，为什么都说这题和树的重心有关

#include <bits/stdc++.h>

using namespace std;

inline int read() {
    char c;
    bool flag = false;
    while ((c = getchar()) < '0' || c > '9') if (c == '-') flag = true;
    int res = c - '0';
    while ((c = getchar()) >= '0' && c <= '9') res = (res << 3) + (res << 1) + c - '0';
    return flag ? -res : res;
}

const int maxn = 110;
int n, dp[maxn], f[maxn], a[maxn], sum[maxn];
struct Edge {
    int to, w, next;
} edge[maxn << 1];
int head[maxn << 1], cnt;

void init() {
    for (int i = 0; i <= n; i++) head[i] = -1;
    cnt = 0;
}

void add_edge(int u, int v, int w) {
    edge[cnt].to = v;
    edge[cnt].w = w;
    edge[cnt].next = head[u];
    head[u] = cnt++;
}

void dfs(int x, int fa) {
    dp[x] = 0;
    sum[x] = a[x];
    for (int i = head[x]; ~i; i = edge[i].next) {
        int y = edge[i].to;
        if (y == fa) continue;
        dfs(y, x);
        dp[x] += dp[y] + sum[y];
        sum[x] += sum[y];
    }
}

void rfs(int x, int fa) {
    for (int i = head[x]; ~i; i = edge[i].next) {
        int y = edge[i].to;
        if (y == fa) continue;
        f[y] = f[x] + sum[1] - 2 * sum[y];
        rfs(y, x);
    }
}

int main() {
    n = read();
    init();
    for (int i = 1; i <= n; ++i) {
        a[i] = read();
        int x = read(), y = read();
        if (x) add_edge(x, i, 1), add_edge(i, x, 1);
        if (y) add_edge(y, i, 1), add_edge(i, y, 1);
    }
    dfs(1, -1);
    f[1] = dp[1];
    rfs(1, -1);
    int ans = 0x3f3f3f3f;
    for (int i = 1; i <= n; ++i) ans = min(ans, f[i]);
    printf("%d", ans);
    return 0;
}