Distributing 0s and 1s in even and odd positions

Given an array of 0s and 1s, write an algo such that,
- the 0s will occupy the even positions.
- the 1s will occupy the odd positions.
- any extra 0s or 1s should be placed at the end of array
Complexity: linear, in a single pass

Answer:

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

//
//  1 0 1 0 1 1 1
//  i j --> move by step of 2
void Distribute0s1s(vector& a, size_t n)
{
    size_t i = 0, // i will point to the even indices
           j = i+1; // j will point to the odd indices
    for ( ; (i < n) && (j < n) ; )
    {
        // already 0 in even position, move ahead
        if ( a[i] == 0 )
        {
            i += 2;
        }

        // already 1 in odd position, move ahead
        if ( a[j] == 1 )
        {
            j += 2;
        }

        if ( i >=n ) break;
        if ( j >=n ) break;

        // 1 in even pos, 0 in odd pos, swap
        if ( a[i] && !a[j] )
        {
            swap( a[i], a[j] );
            i += 2;
            j += 2;
        }
    }

    if ( i < n )
    {
        // At this point, all the odd positions are enused with 
        // 1s
        j = i + 2;
    }
    else if ( j < n )
    {
        // At this point, all the even positions are enused with 
        // 0s
        i = j;
        j = i + 2;
    }

    // handle the remaining numbers
    while ( j < n )
    {
        if (
                (  ( i % 2 == 0 ) &&  ( a[j] < a[i] )  ) ||
                (  ( i % 2 == 1 ) &&  ( a[j] > a[i] )  )
           )
        {
            swap( a[j], a[i] );
            i += 2;
        }
        j += 2;
    }
}

int main( int argc, char* argv[] )
{
    if ( argc == 1 ) return 0;
    else
    {
        vector a(argc-1);
        for ( int i = 1; i <= argc-1; ++i)
            a[i-1] = atoi(argv[i]);
        cout << "Input :" << endl;
        copy ( a.begin(), a.end(), ostream_iterator(cout, " "));
        cout << endl << "Output :" << endl;
        Distribute0s1s(a, a.size() );
        copy ( a.begin(), a.end(), ostream_iterator(cout, " "));
    }
    return 0xabcdef;

}