sort even numbers ascending then odd numbers descending

Question

i have to use std::sort() to sort numbers in an array like this:

even numbers ascending and then odd numbers descending

so far i got:

#include <algorithm>
#include <iostream>                                 
#include <vector>                                       

using namespace std;


bool order(int a, int b)                                    
{
    if (a % 2 == 0)
    {
        return a < b;
    }
    else if(a % 2 != 0)
    {
        return b < a;
    }
}
int main()                                              
{
    std::vector<int> v = { 2, 3, 5, 6, 4, 1 };          
    std::sort(v.begin(), v.end(), order);               

}

but i cant figure out the correct order algorithm to do so

Answer 1

if (a % 2 == b % 2) { // same parity
    if (a % 2) { // odd descending
         return b < a; 
     } else { // even ascending
         return a < b;
     }
} else { // odd is bigger than even
    return b % 2;
}

Answer 2

With std:

std::vector<int> v {2, 3, 5, 6, 4, 1};
auto it = std::partition(v.begin(), v.end(), [](int e) { return e % 2 == 0; });

std::sort(v.begin(), it);
std::sort(it, v.end(), std::greater<>{});

// `v` would be now { 2, 4, 6, 5, 3, 1}.