1 bool func(int n)
 2 {
 3     return (n&1)==0; // n%2 is more expensive
 4 }
 5 
 6 void group_oddeven(std::vector<int>& a, bool (*func)(int))
 7 {
 8     int i = 0, j = a.size()-1;
 9     int buf = 0;
10     while (i < j) {
11         if (!func(a[i]))    // odd, move forward
12         {   i++; continue; }
13         if (func(a[j]))     // even, move backward
14         {   j--; continue; }
15 
16         std::swap(a[i++], a[j--]);
17     }
18 }
19