This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
/// #pragma GCC optimize ("Ofast")
/// #pragma GCC target ("avx2")
/// #pragma GCC optimize("unroll-loops")
using namespace std;
using ll = long long;
using ld = long double;
#define ff first
#define ss second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define lb lower_bound
const int MOD = 998244353;
int N, A, B;
vector<int> adj[300005];
int DP[300005];
void dfs(int v, int p) {
int C = 1;
for(auto u : adj[v]) {
if(u == p) continue;
if(DP[u] > DP[v] + C) {
DP[u] = min(DP[u], DP[v] + C);
++C;
}
dfs(u, v);
}
}
int32_t main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> N >> A >> B;
for(int l = 0; l < N - 1; l++) {
int U, V; cin >> U >> V;
adj[U].push_back(V);
adj[V].push_back(U);
}
for(int l = 1; l <= N; l++) DP[l] = 2e9;
DP[A] = 0; dfs(A, A);
DP[B] = 0; dfs(B, B); int res = 0;
for(int l = 1; l <= N; l++)
res = max(res, DP[l]);
cout << res << "\n";
return 0;
}
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