usaco 2.1 sort3...想法题...

/*
    ID:chenjiong
    PROG:sort3
    LANG:C++
*/

#include <stdio.h>
#include <string.h>

const int MAXN = 1005;

int N;
int a[MAXN];
int p1,p2,p3;
int cnt;

void swap(int& a,int& b)
{
    a = a + b;
    b = a - b;
    a = a - b;
}

void init()
{
    scanf("%d",&N);
    p1 = p2 = p3 = cnt = 0;

    int i;
    for ( i = 0; i < N; i++)
    {
        scanf("%d",&a[i]);
        if ( a[i] == 1 )
            p1++;
        else if ( a[i] == 2 )
            p2++;
        else
            p3++;
    }
}

void search()
{
    int i,j;
    for ( i = 0; i <= p1 - 1; i++)
    {
        if ( a[i] == 1 )
            continue;
        else if ( a[i] == 2 )
        {
            for ( j = p1; j < N; j++)
            {
                if ( a[j] == 1 )
                {
                    swap(a[i],a[j]);
                    cnt++;
                    break;
                }
            }
        }
        else 
        {
            for ( j = N - 1; j >= p1; j--)
            {
                if ( a[j] == 1 )
                {
                    swap(a[i],a[j]);
                    cnt++;
                    break;
                }
            }
        }
    }

    for ( i = p1; i <= p1 + p2 - 1; i++)
    {
        if ( a[i] == 3 )
        {
            for ( j = N - 1; j >= p1 + p2; j--)
            {
                if ( a[j] == 2 )
                {
                    swap(a[i],a[j]);
                    cnt++;
                    break;
                }
            }
        }
    }
}

int main()
{
    freopen("sort3.in","r",stdin);
    freopen("sort3.out","w",stdout);

    init();
    search();

    printf("%d\n",cnt);

    return 0;
}