徐州 icpc 區域賽 M. Kill the tree 樹的重心

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define inf 0x3f3f3f3f
const int mx = 2e5+100;
vector<int>ve[mx];
int so[mx], sou[mx], sum[mx],  n, h[mx];
int ff[mx];


void solve(int x, int u, int y) {
    while (x != u && so[u]-so[x] > so[x]) {
        x = ff[x];
    }
    while (y != u && so[u]-so[y] > so[y]) {
        y = ff[y];
    }
    if (h[x] > h[y]) sou[u] = x;
    else sou[u] = y;
    return;
}

void dfs1(int u, int fa) {
    sou[u] = u;
    so[u] = 1, ff[u] = fa;
    for (int i = 0; i < ve[u].size(); ++i) {
        int v = ve[u][i];
        if (v == fa) continue;
        h[v] = h[u]+1;
        dfs1(v, u);
        so[u] += so[v];
        solve(sou[u], u, sou[v]);
    }
}

int main(){
    int u, v;
    scanf("%d", &n);
    for (int i = 1; i < n; ++i) {
        scanf("%d%d", &u, &v);
        ve[u].push_back(v);
        ve[v].push_back(u);
    }
    dfs1(1, 0);
    for (int i = 1; i <= n; ++i) {
        int a = sou[i], b = -1;
        if (sou[i] != i) {
//            printf("%d %d\n", so[i]-so[sou[i]]-1, so[ff[sou[i]]]);
            if (so[i]-so[sou[i]] == so[sou[i]]) b = ff[sou[i]];
            if (b != -1) {
                if (a > b) swap(a, b);
            }
        }
        printf("%d", a);
        if (b != -1) printf(" %d", b);
        puts("");
    }
}