换根DP

P3478

#include<bits/stdc++.h>
using namespace std;
#define f(i, a, b) for(int i = (a); i <= (b); i++)
#define cl(i, n) i.clear(),i.resize(n);
#define endl '\n'
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
const int inf = 1e9;
ll n;
vector<int> g[1000010];
ll sz[1000010], dp[1000010];
ll mx; int ans;
void dfs1(int now, int fa) {
    sz[now] = 1;
    f(i, 0, g[now].size() - 1) {
        if(g[now][i] != fa){
            dfs1(g[now][i], now); sz[now] += sz[g[now][i]];
        }
    }
}
void dfs2(int now, int fa) {
    f(i, 0, g[now].size() - 1) {
        if(g[now][i] != fa) {
            dp[g[now][i]] = dp[now] + n - 2 * sz[g[now][i]];
            if(dp[g[now][i]] > mx) {
                mx = dp[g[now][i]]; ans = g[now][i];
            }
            dfs2(g[now][i], now); 
        }
    }
}
int main() {
    ios::sync_with_stdio(0);
    cin.tie(NULL);
    cout.tie(NULL);
    cin >> n;
    f(i, 1, n - 1) {
        int x, y; cin >> x >> y;
        g[x].push_back(y); g[y].push_back(x);
    }
    dfs1(1, 0); f(i, 2, n) dp[1] += sz[i];
    dfs2(1, 0); cout << ans << endl;
    return 0;
}

标签：sz,int,ll,fa,DP,now,换根,dp
来源： https://www.cnblogs.com/Zeardoe/p/16462910.html