思路：随便想想就能想出来啦把。。。  卡了我一个vector。。。

#include
     
      #define LL long long#define fi first#define se second#define mk make_pair#define pii pair
      
       #define piii pair
       
         >using namespace std;const int N = 1e6 + 10;const int M = 10 + 7;const int inf = 0x3f3f3f3f;const LL INF = 0x3f3f3f3f3f3f3f3f;const int mod = 1e9 + 7;const double eps = 1e-6;int n, tot, head[N];LL cnt[N], sum[N], dp[N];struct Edge {    int to, nx;} edge[N << 1];void add(int u, int v) {    edge[tot].to = v;    edge[tot].nx = head[u];    head[u] = tot++;}void dfs1(int u, int p) {    cnt[u] = 1;    for(int i = head[u]; ~i; i = edge[i].nx) {        int v = edge[i].to;        if(v == p) continue;        dfs1(v, u);        cnt[u] += cnt[v];        sum[u] += sum[v] + cnt[v];    }}void dfs2(int u, int p) {    if(p) {        dp[u] = sum[u] + (dp[p] - (sum[u] + cnt[u]) + n - cnt[u]);    }    for(int i = head[u]; ~i; i = edge[i].nx) {        int v = edge[i].to;        if(v == p) continue;        dfs2(v, u);    }}int main() {    memset(head, -1, sizeof(head));    scanf("%d", &n);    for(int i = 1; i < n; i++) {        int u, v; scanf("%d%d", &u, &v);        add(u, v);        add(v, u);    }    dfs1(1, 0);    dp[1] = sum[1];    dfs2(1, 0);    int ans = 1;    for(int i = 1; i <= n; i++) {        if(dp[ans] < dp[i]) {            ans = i;        }    }    printf("%d\n", ans);    return 0;}/**/