飞行员配对方案问题

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
int n,m,hed[105],cnt;
struct EG
{
    int to,nxt;
}e[10005];
void ae(int f,int t)
{
    e[++cnt].to = t;
    e[cnt].nxt = hed[f];
    hed[f] = cnt;
}
int fa[105],vis[105];
bool dfs(int u)
{
    vis[u]=1;
    for(int j=hed[u];j;j=e[j].nxt)
    {
        int to = e[j].to;
        if(vis[to])continue;
        vis[to]=1;
        if(!fa[to]||dfs(fa[to]))
        {
            fa[to]=u;
            return 1;
        }
    }
    return 0;
}
struct ass
{
    int x,y;
}as[102];
int tot;
bool cmp(ass a,ass b)
{
    return a.x<b.x;
}
int main()
{
    scanf("%d%d",&n,&m);
    int f,t;
    while(1)
    {
        scanf("%d%d",&f,&t);
        if(f==-1&&t==-1)break;
        ae(f,t);
    }
    int ans = 0;
    for(int i=1;i<=n;i++)
    {
        memset(vis,0,sizeof(vis));
        if(dfs(i))
            ans++;
    }
    printf("%d\n",ans);
    for(int i=n+1;i<=m;i++)
    {
        if(fa[i])
        {
            as[++tot].x=fa[i];
            as[tot].y=i;
        }
    }
    sort(as+1,as+1+tot,cmp);
    for(int i=1;i<=tot;i++)
        printf("%d %d\n",as[i].x,as[i].y);
    return 0;
}