[SCOI2010]游戏

题目地址：

https://ac.nowcoder.com/acm/problem/20566

基本思路：

#pragma GCC optimize(2)
#pragma GCC optimize(3)
#include <bits/stdc++.h>
using namespace std;
#define IO std::ios::sync_with_stdio(false); cin.tie(0)
#define int long long
#define SZ(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rep(i, l, r) for (int i = l; i <= r; i++)
#define per(i, l, r) for (int i = l; i >= r; i--)
#define mset(s, _) memset(s, _, sizeof(s))
#define pb push_back
#define pii pair <int, int>
#define mp(a, b) make_pair(a, b)
#define INF 0x3f3f3f3f

inline int read() {
  int x = 0, neg = 1; char op = getchar();
  while (!isdigit(op)) { if (op == '-') neg = -1; op = getchar(); }
  while (isdigit(op)) { x = 10 * x + op - '0'; op = getchar(); }
  return neg * x;
}
inline void print(int x) {
  if (x < 0) { putchar('-'); x = -x; }
  if (x >= 10) print(x / 10);
  putchar(x % 10 + '0');
}

const int maxn = 1e6 + 10;
struct Edge{
    int to,next;
}edge[maxn << 1];
int n,cnt,head[maxn];
int now,use[maxn],match[maxn];
void add_edge(int u,int v) {
  edge[++cnt].next = head[u];
  edge[cnt].to = v;
  head[u] = cnt;
}
bool dfs(int u) { // 二分图匹配匈牙利算法;
  for (int i = head[u]; i != -1; i = edge[i].next) {
    int to = edge[i].to;
    if (use[to] == now) continue;
    use[to] = now;
    if (!match[to] || dfs(match[to])) {
      match[to] = u;
      return true;
    }
  }
  return false;
}
signed main() {
  IO;
  cin >> n;
  cnt = 0,mset(head,-1);
  rep(i,1,n) {
    int x, y;
    cin >> x >> y;
    add_edge(x, i);
    add_edge(y, i);
  }
  int ans = 0;
  for(int i = 1 ; i <= 10000 ; i++){
    now = i; // 标记use的时间戳,代替memset;
    if(dfs(i)) ans++;
    else break;
  }
  cout << ans << '\n';
  return 0;
}