ZOJ 4033 CONTINUE...?（The 15th Zhejiang Provincial Collegiate Programming Contest Sponsored by TuSimple）

#include <iostream>
#include <algorithm>
using namespace std;

const int maxn = 1e5 + 5;
int a[maxn];

int main(){
    int t;
    cin >> t;
    while(t--){
        int n;
        cin >> n;
        string s;
        cin >> s;
        int sum = 0;
        bool l = true;
        int i;
        for(i = n-1;i >= 1;i -= 2){
            sum += 2*i+1;
            if(l){
                if(s[i] == '0'){
                    a[i] = 2;
                }
                else
                    a[i] = 4;
                if(s[i-1] == '0'){
                    a[i-1] = 1;
                }
                else a[i-1] = 3;
                l = false;
            }
            else{
                if(s[i] == '0'){
                    a[i] = 1;
                }
                else
                    a[i] = 3;
                if(s[i-1] == '0'){
                    a[i-1] = 2;
                }
                else a[i-1] = 4;
                l = true;
            }
        }
        if(i == 0){
            sum += 1;
            if(l){
                if(s[0] == '0'){
                    a[0] = 2;
                }
                else
                    a[0] = 4;
            }
            else{
                if(s[0] == '0'){
                    a[0] = 1;
                }
                else
                    a[0] = 3;
            }
        }

        if(sum%2){
            cout << -1 << endl;
            continue;
        }
        else{
            for(int j = 0;j < n;j++){
                cout << a[j] ;
            }
            cout << endl;
        }

    }
    return 0;
}