codeforces 158B

http://codeforces.com/problemset/problem/158/B

贪心：4人1车，（3+1）人1车，（2+2）人1车，（2+1+1）人1车，（1*4）人1车

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
int a[5];
int main()
{
   // freopen("1.txt","r",stdin);
    int i,n,x,sum;
    while(~scanf("%d",&n))
    {
        memset(a,0,sizeof(a));
        sum=0;
        for(i=0; i<n; i++)
        {
            cin>>x;
            a[x]++;
        }
        sum+=a[4]+a[3];
        x=a[1]-a[3];
        if(a[2]%2==0)
            sum+=a[2]/2;
        else
        {
            sum+=(a[2]+1)/2;
            x-=2;
        }
        if(x>0)
        {
            sum+=x/4;
            x=x%4;
            if(x)
            sum++;
        }
        cout<<sum<<endl;
    }
    return 0;
}