[SCOI 2010] 连续攻击游戏

[题目链接]

          https://www.luogu.org/problemnew/show/P1640

[算法]

          二分图匹配 

          实现时需要常数优化和特判

[代码]

        

//code by byf and lmj
#include <bits/stdc++.h>
#define MAXN 2000005
using namespace std;
int n,tot=0,match[MAXN],head[MAXN];
bool visited[MAXN];
struct edge
{
    int to,nxt;    
} e[MAXN << 1];
template <typename T> inline void read(T &x)
{
    int f = 1; x = 0;
    char c = getchar();
    for (; !isdigit(c); c = getchar())
    {
        if (c == '-') f = -f;
    }
    for (; isdigit(c); c = getchar()) x = (x << 3) + (x << 1) + c - '0';
    x *= f; 
} 
inline void addedge(int u,int v)
{
    tot++;
    e[tot] = (edge){v,head[u]};
    head[u] = tot;
}
inline bool hungary(int u)
{
    int v;
    visited[u] = true;
    for (register int i = head[u]; i; i = e[i].nxt)
    {
        v = e[i].to;
        if (!visited[v]) 
        {
            visited[v] = true;
            if (!match[v] || hungary(match[v]))
            {
                match[v] = u;
                return true;
            }
        } 
    } 
    return false;
}
int main() {
    read(n);
    for (register int i = 1; i <= n; ++i)
    {
        int a,b;
        read(a); read(b);
        addedge(a,i+n); addedge(b,i+n);
    }
    if (n <= 5E4)
    {
        for (register int i = 1; i <= n; ++i)
        {
            memset(visited,false,sizeof(visited));
            if (!hungary(i))
            {
                cout << i - 1 << endl;
                return 0;
            }
        }
        cout << n << endl;
        return 0;
    }
    for (register int i = 1; i <= 10005; ++i)
    {
        for (register int j = 1; j <= 20005; ++j) visited[j] = false;
        if (!hungary(i))
        {
            cout << i - 1 << endl;
            return 0;
        }
    }
    cout << n << endl;
    return 0;
}