#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
using namespace std;
// using namespace __gnu_pbds;
// template<class T>
// using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template<class T, class U>
void ckmin(T &a, U b)
{
if (a > b) a = b;
}
template<class T, class U>
void ckmax(T &a, U b)
{
if (a < b) a = b;
}
#define MP make_pair
#define PB push_back
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
#define SZ(x) ((int) (x).size())
#define ALL(x) (x).begin(), (x).end()
#define FOR(i, a, b) for (auto i = (a); i < (b); i++)
#define FORD(i, a, b) for (auto i = (a) - 1; i >= (b); i--)
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<pii> vpi;
typedef vector<pll> vpl;
const int MAXN = 200013;
int N;
vi edge[MAXN];
int parent[MAXN], subtree[MAXN];
set<int> sizes[MAXN];
int ans;
vi nums;
void dfs(int u, int p)
{
subtree[u] = 1;
for (int v : edge[u])
{
if (v == p) continue;
parent[v] = u;
dfs(v, u);
subtree[u] += subtree[v];
}
}
//case 0: they're in same subtree
int calc(int a, int b)
{
int c = N - a - b;
return max(a, max(b, c)) - min(a, min(b, c));
}
int solve(set<int> &a, set<int> &b)
{
for (int x : a)
{
auto it = b.LB((N - x) / 2);
if (it != b.end())
{
ckmin(ans, calc(x, *it));
}
if (it != b.begin())
{
ckmin(ans, calc(x, *prev(it)));
}
}
return;
}
//case 1: they're in two separate subtrees
void solve1(int u)
{
for (int v : edge[u])
{
if (v == parent[u]) continue;
solve1(v);
}
for (int v : edge[u])
{
if (subtree[v] > SZ(sizes[u])) swap(sizes[u], sizes[v]);
ckmin(ans, solve(sizes[u], sizes[v]));
for (int x : sizes[v]) sizes[u].insert(x);
}
sizes[u].insert(subtree[u]);
}
void solve2(int u)
{
nums.PB(subtree[u]);
for (int v : edge[u])
{
if (v == parent[u]) continue;
solve2(v);
}
nums.pop_back();
int sz = (N + subtree[u]) / 2;
auto it = LB(ALL(nums), sz);
if (it != nums.end())
{
ckmin(ans, calc(subtree[u], *it));
}
if (it != nums.begin())
{
ckmin(ans, calc(subtree[u], *prev(it)));
}
return;
//now solve: one cut is subtree[u], the other cut is smth before
}
int32_t main()
{
cout << fixed << setprecision(12);
cerr << fixed << setprecision(4);
ios_base::sync_with_stdio(false); cin.tie(0);
cin >> N;
FOR(i, 0, N - 1)
{
int u, v;
cin >> u >> v; u--; v--;
edge[u].PB(v);
edge[v].PB(u);
}
parent[0] = N;
dfs(0, N);
ans = N;
solve2(0);
solve1(u);
cout << ans << '\n';
return 0;
}
Compilation message
papricice.cpp: In function 'int solve(std::set<int>&, std::set<int>&)':
papricice.cpp:84:5: error: return-statement with no value, in function returning 'int' [-fpermissive]
84 | return;
| ^~~~~~
papricice.cpp: In function 'int32_t main()':
papricice.cpp:142:12: error: 'u' was not declared in this scope
142 | solve1(u);
| ^
