[Codeforces Round49F] Session in BSU

[题目链接]

http://codeforces.com/contest/1027/problem/F

[算法]

二分图匹配

[代码]

#include<bits/stdc++.h>
#pragma GOC optimize("Ofast")
using namespace std;
const int MAXN = 1e6 + ;

struct edge
{
        int to,nxt;
} e[MAXN << ];

int n,q,len,ans,tot;
int a[MAXN],b[MAXN],ta[MAXN],tb[MAXN],tmp[MAXN << ],match[MAXN << ],head[MAXN << ],visited[MAXN << ];

template <typename T> inline void read(T &x)
{
        int f = ; x = ;
        char c = getchar();
        for (; !isdigit(c); c = getchar()) if (c == '-') f = -f;
        for (; isdigit(c); c = getchar()) x = (x << ) + (x << ) + c - '';
        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 k)
{
        int v;
        for (int i = head[u]; i; i = e[i].nxt)
        {
                v = e[i].to;
                if (visited[v] != k)
                {
                        visited[v] = k;
                        if (!match[v] || hungary(match[v],k))
                        {
                                match[v] = u;
                                return true;
                        }
                }
        }
        return false;
}

int main()
{

        read(n);
        for (int i = ; i <= n; i++)
        {
                read(a[i]); read(b[i]);
                tmp[++len] = a[i]; tmp[++len] = b[i];
        }
        sort(tmp + ,tmp + len + );
        len = unique(tmp + ,tmp + len + ) - tmp - ;
        for (int i = ; i <= n; i++)
        {
                ta[i] = lower_bound(tmp + ,tmp + len + ,a[i]) - tmp;
                tb[i] = lower_bound(tmp + ,tmp + len + ,b[i]) - tmp;
        }
        for (int i = ; i <= n; i++)
        {
                addedge(ta[i],len + i);
                addedge(tb[i],len + i);
        }
        for (int i = ; i <= len; i++)
        {
                if (hungary(i,i))
                {
                        ans++;
                        if (ans == n)
                        {
                                printf("%d\n",tmp[i]);
                                return ;
                        }
                }
        }
        printf("-1\n");

        return ;

}