Problem p03055 (C++)

Buggy submission (status: Runtime Error):
// Hacheylight
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
using namespace std;
// #define int long long
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define loop(it, v) for (auto it = v.begin(); it != v.end(); it++)
#define cont(i, x) for (int i = head[x]; i; i = e[i].nxt)
#define clr(a) memset(a, 0, sizeof(a))
#define ass(a, sum) memset(a, sum, sizeof(a))
#define lowbit(x) (x & -x)
#define all(x) x.begin(), x.end()
#define SC(t, x) static_cast<t>(x)
#define ub upper_bound
#define lb lower_bound
#define pqueue priority_queue
#define mp make_pair
#define pb push_back
#define pof pop_front
#define pob pop_back
#define fi first
#define se second
#define y1 y1_
#define Pi acos(-1.0)
#define iv inline void
#define enter cout << endl
#define siz(x) ((int)x.size())
#define file(x) freopen(x ".in", "r", stdin), freopen(x ".out", "w", stdout)
typedef double db;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pii> vii;
typedef queue<int> qi;
typedef queue<pii> qii;
typedef set<int> si;
typedef map<int, int> mii;
typedef map<string, int> msi;
const int N = 100010;
const int INF = 0x3f3f3f3f;
const int iinf = 1 << 30;
const ll linf = 2e18;
const int MOD = 1000000007;
const double eps = 1e-7;
void douout(double x) { printf("%lf\n", x + 0.0000000001); }
template <class T> void print(T a) {
  cout << a << endl;
  exit(0);
}
template <class T> void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
template <class T> void chmax(T &a, T b) {
  if (a < b)
    a = b;
}
template <class T> void add(T &a, T b) { a = (1ll * a + b) % MOD; }
template <class T> void mul(T &a, T b) { a = (ll)a * b % MOD; }
template <class T> T read() {
  int f = 1;
  T x = 0;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-')
      f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}

// pair <从端点开始的顶点的个数, 端点>

vi g[N];
int n;

pii dfs(int x, int fat) {
  pii ans = mp(0, x);
  for (int to : g[x]) {
    if (to == fat)
      continue;
    chmax(ans, dfs(to, x));
  }
  ans.fi++;
  return ans;
}

signed main() {
  scanf("%d", &n);
  rep(i, 1, n - 1) {
    int a, b;
    scanf("%d%d", &a, &b);
    g[a].pb(b);
    g[b].pb(a);
  }
  int dia = dfs(dfs(1, -1).se, -1).fi;
  puts((dia % 3 != 2) ? "First" : "Second");
  return 0;
}

/*
写代码时请注意：
        1.ll？数组大小，边界？数据范围？
        2.精度？
        3.特判？
        4.至少做一些
思考提醒：
        1.最大值最小->二分？
        2.可以贪心么？不行dp可以么
        3.可以优化么
        4.维护区间用什么数据结构？
        5.统计方案是用dp？模了么？
        6.逆向思维？
*/


Fixed submission:
// Hacheylight
#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
using namespace std;
// #define int long long
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define loop(it, v) for (auto it = v.begin(); it != v.end(); it++)
#define cont(i, x) for (int i = head[x]; i; i = e[i].nxt)
#define clr(a) memset(a, 0, sizeof(a))
#define ass(a, sum) memset(a, sum, sizeof(a))
#define lowbit(x) (x & -x)
#define all(x) x.begin(), x.end()
#define SC(t, x) static_cast<t>(x)
#define ub upper_bound
#define lb lower_bound
#define pqueue priority_queue
#define mp make_pair
#define pb push_back
#define pof pop_front
#define pob pop_back
#define fi first
#define se second
#define y1 y1_
#define Pi acos(-1.0)
#define iv inline void
#define enter cout << endl
#define siz(x) ((int)x.size())
#define file(x) freopen(x ".in", "r", stdin), freopen(x ".out", "w", stdout)
typedef double db;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pii> vii;
typedef queue<int> qi;
typedef queue<pii> qii;
typedef set<int> si;
typedef map<int, int> mii;
typedef map<string, int> msi;
const int N = 200010;
const int INF = 0x3f3f3f3f;
const int iinf = 1 << 30;
const ll linf = 2e18;
const int MOD = 1000000007;
const double eps = 1e-7;
void douout(double x) { printf("%lf\n", x + 0.0000000001); }
template <class T> void print(T a) {
  cout << a << endl;
  exit(0);
}
template <class T> void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
template <class T> void chmax(T &a, T b) {
  if (a < b)
    a = b;
}
template <class T> void add(T &a, T b) { a = (1ll * a + b) % MOD; }
template <class T> void mul(T &a, T b) { a = (ll)a * b % MOD; }
template <class T> T read() {
  int f = 1;
  T x = 0;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-')
      f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}

// pair <从端点开始的顶点的个数, 端点>

vi g[N];
int n;

pii dfs(int x, int fat) {
  pii ans = mp(0, x);
  for (int to : g[x]) {
    if (to == fat)
      continue;
    chmax(ans, dfs(to, x));
  }
  ans.fi++;
  return ans;
}

signed main() {
  scanf("%d", &n);
  rep(i, 1, n - 1) {
    int a, b;
    scanf("%d%d", &a, &b);
    g[a].pb(b);
    g[b].pb(a);
  }
  int dia = dfs(dfs(1, -1).se, -1).fi;
  puts((dia % 3 != 2) ? "First" : "Second");
  return 0;
}

/*
写代码时请注意：
        1.ll？数组大小，边界？数据范围？
        2.精度？
        3.特判？
        4.至少做一些
思考提醒：
        1.最大值最小->二分？
        2.可以贪心么？不行dp可以么
        3.可以优化么
        4.维护区间用什么数据结构？
        5.统计方案是用dp？模了么？
        6.逆向思维？
*/