int repeatNum(vector<int>& v){
    if(v.empty()) return -1;
    int len = v.size();
    sort(v.begin(), v.end());
    for(int i = 1; i < len; i++){
        if(v[i] == v[i-1]){
            return v[i];
        }
    }
    return -1;
}

int repeatNum(vector<int>& v){
    if(v.empty()) return -1;
    map<int, int> m;
    for(int i = 0; i < v.size(); i++){
        if(m[v[i]]) return v[i];
        else m[v[i]]++;
    }
    return -1;
}

int repeatNum(vector<int>& v){
    if(v.empty()) return -1;
    for(int i = 0; i < v.size(); ++i){
        if(v[i] < 0 || v[i] > v.size()-1) // 數(shù)字必須在 0 ~ n-1 之間
            return -1;
    }
    
    for(int i = 0; i < v.size(); ++i){
        while(v[i] != i){
            if(v[i] == v[v[i]]) return v[i];
            swap(v[i], v[v[i]]);
        }
    }
    return false;
}

int repeatNum(vector<int>& v){
    if(v.empty()) return -1;
    map<int, int> m;
    for(int i = 0; i < v.size(); i++){
        if(m[v[i]]) return v[i];
        else m[v[i]]++;
    }
    return -1;
}

int repeatNum(vector<int>& v){
    int len = v.size();
    vector<int> v1(len);
    for(int i = 0; i < len; ++i){
        if(v1[v[i]]) return v1[v[i]];
        else v1[v[i]] = v[i];
    }
    return -1;
}

int countRange(vector<int>& v, int sz, int start, int end){
    if(v.empty()) return 0;

    int count = 0;
    for(int i = 0; i < sz; ++i){
        if(v[i] >= start && v[i] <= end){
            ++count;
        }
    }
    return count;
}

int getrepeat(vector<int>& v){
    if(v.empty()) return -1;
    
    int sz = v.size();
    int start = 1, end = sz-1;
    while(end >= start){
        int mid = start + ((end-start)>>1);
        int count = countRange(v, sz, start, mid);
        if(end == start){
            if(count > 1) return start;
            else break;
        }
        if(count > (mid - start + 1)) end = mid;
        else start = mid + 1;
    }
    return -1;
}

bool duplicate(vector<int>& v, int **res){
    if(v.empty()) return false;
    for(int i = 0; i < v.size(); ++i){
        if(v[i] < 0 || v[i] > v.size()-1) 
            return false;
    }
    
    for(int i = 0; i < v.size(); ++i){
        while(v[i] != i){
            if(v[i] == v[v[i]]) {
                *res = &v[i];
                return true;
            }
            swap(v[i], v[v[i]]);
        }
    }
    return false;
}

int main(){
    int arr[] = {2, 3, 5, 4, 3, 2, 6, 7};
    vector<int> v(arr, arr+8);  // 這種賦值方式不會導致vector自動擴展內部大小

    int* res = nullptr;
    if(duplicate(v, &res)) cout << *res << endl;
    else cout << '0' << endl;
    //cout << repeatNum(v) << endl;
    /*
    for(int i = 0; i < v.size(); i++){
        if(i == 0) cout << v[i];
        else cout << ' ' << v[i];
    }
    cout << endl;
    
    for(auto a : v){    // 有兩個警告（auto是C++_11的擴展）
        cout << a << ' ';
    }
    */
    return 0;
}