BOJ 2138 전구와 스위치

void solve(){
    int N; cin >> N;
    string S, T; cin >> S >> T;
    string ret = "";
    rep(i, 0, N) ret += (S[i] == T[i] ? "0" : "1");

    int ans = 1e9;

    // don't flip first
    string tmp = ret;
    int cnt = 0;
    rep(i, 1, N){
        if(tmp[i-1] == '1'){
            cnt++;
            tmp[i-1] = (tmp[i-1] == '1' ? '0' : '1');
            tmp[i] = (tmp[i] == '1' ? '0' : '1');
            if(i+1 < N) tmp[i+1] = (tmp[i+1] == '1' ? '0' : '1');
        }
    }
    if(tmp[N-1] == '0') ans = min(ans, cnt);

    // flip first
    tmp = ret;
    cnt = 1;
    tmp[0] = (tmp[0] == '1' ? '0' : '1');
    tmp[1] = (tmp[1] == '1' ? '0' : '1');
    rep(i, 1, N){
        if(tmp[i-1] == '1'){
            cnt++;
            tmp[i-1] = (tmp[i-1] == '1' ? '0' : '1');
            tmp[i] = (tmp[i] == '1' ? '0' : '1');
            if(i+1 < N) tmp[i+1] = (tmp[i+1] == '1' ? '0' : '1');
        }
    }
    if(tmp[N-1] == '0') ans = min(ans, cnt);

    if(ans == 1e9) ans = -1;
    cout << ans << "\n";
}