CONTINUE...? ZOJ - 4033

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <queue>
using namespace std;
typedef long long ll;
const ll maxn = 100005;
char a[maxn];
ll b[maxn];
int main()
{
    ll t,n,i,j;
    scanf("%lld",&t);
    while(t--)
    {
        scanf("%lld %s",&n, a + 1);
        ll sum = (n * (n + 1)) / 2;
        if(sum % 2 != 0)
        {
            printf("-1\n");
            continue;
        }
        memset(b,0,sizeof(b));
        sum  = sum / 2;
        for(i = n; i >= 1; i --)
        {
            if(sum >= i)
            {
                sum -= i;
                if(a[i]  == '0')
                    b[i] = 1;
                else
                    b[i] = 3;
                if(sum == 0) break;
            }
            else{
                if(a[sum] == '0') b[sum] = 1;
                else b[sum] = 3;
                break;
            }
        }
        for(i = 1; i <= n; i ++)
        {
            if(b[i] == 0)
            {
                if(a[i] == '0')
                    printf("2");
                else
                    printf("4");
            }
            else printf("%lld",b[i]);
        }
        printf("\n");
    }
    return 0;
}