题解 | #桃花#

求树的直径

#include <bits/stdc++.h>

#define x first
#define y second
#define all(x) x.begin(), x.end()
#define vec1(T, name, n, val) vector<T> name(n, val)
#define vec2(T, name, n, m, val) vector<vector<T>> name(n, vector<T>(m, val))
#define vec3(T, name, n, m, k, val) vector<vector<vector<T>>> name(n, vector<vector<T>>(m, vector<T>(k, val)))
#define vec4(T, name, n, m, k, p, val) vector<vector<vector<vector<T>>>> name((n), vector<vector<vector<T>>>((m), vector<vector<T>>((k), vector<T>((p), (val)))))

using namespace std;
using i128 = __int128;
using u128 = unsigned __int128;
using LL = long long;
using LD = long double;
using ULL = unsigned long long;
using PII = pair<int, int>;
using PLL = pair<LL, LL>;
using PLD = pair<LD, LD>;

const int N = 1e5 + 10, MOD = 998244353;
const int INF = 1e9;
const LL LL_INF = 2e18;
const LD EPS = 1e-8;
const int dx4[] = {-1, 0, 1, 0}, dy4[] = {0, 1, 0, -1};
const int dx8[] = {-1, -1, -1, 0, 0, 1, 1, 1}, dy8[] = {-1, 0, 1, -1, 1, -1, 0, 1};
const int hx[] = {-2, -2, -1, -1, 1, 1, 2, 2}, hy[] = {-1, 1, -2, 2, -2, 2, -1, 1};

istream& operator>>(istream& is, i128& val) {
    string str;
    is >> str;
    val = 0;
    bool flag = false;
    if (str[0] == '-') flag = true, str = str.substr(1);
    for (char& c : str) val = val * 10 + c - '0';
    if (flag) val = -val;
    return is;
}

ostream& operator<<(ostream& os, i128 val) {
    if (val < 0) os << "-", val = -val;
    if (val > 9) os << val / 10;
    os << static_cast<char>(val % 10 + '0');
    return os;
}

bool cmp(LD a, LD b) {
    if (fabs(a - b) < EPS) return 1;
    return 0;
}

LL qpow(LL a, LL b) {
    LL ans = 1;
    a %= MOD;
    while (b) {
        if (b & 1) ans = ans * a % MOD;
        a = a * a % MOD;
        b >>= 1;
    }
    return ans;
}

void solve() {
    int n;
    cin >> n;
    vector<vector<int>> g(n + 1);
    for (int i = 1; i < n; ++i) {
        int a, b;
        cin >> a >> b;
        g[a].push_back(b);
        g[b].push_back(a);
    }

    vector<int> dep(n + 1, 0);
    dep[1] = 1;
    auto dfs1 = [&](auto dfs1, int u, int fa) -> void {
        for (int &v : g[u]) {
            if (v == fa) continue;
            dep[v] = dep[u] + 1;
            dfs1(dfs1, v, u);
        }
    };

    dfs1(dfs1, 1, 0);

    int p = 1;
    int mx = 1;
    for (int i = 1; i <= n; ++i) {
        if (dep[i] > mx) {
            mx = dep[i];
            p = i;
        }
    }

    fill(all(dep), 0);
    dep[p] = 1;
    auto dfs2 = [&](auto dfs2, int u, int fa) -> void {
        for (int &v : g[u]) {
            if (v == fa) continue;
            dep[v] = dep[u] + 1;
            dfs2(dfs2, v, u);
        }
    };

    dfs2(dfs2, p, 0);

    mx = 1;
    for (int i = 1; i <= n; ++i) mx = max(mx, dep[i]);

    cout << mx << '\n';
/**/ #ifdef LOCAL
    cout << flush;
/**/ #endif
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);

    int T = 1;
    while (T--) solve();
    cout << fixed << setprecision(15);

    return 0;
}