// single threaded pseudo-code

Q=5 // Q is magic number

Sel( k, a, size )
{
    if ( size < Q*Q ) { Sort( a, size); return a[k]; } // Q*Q is magic number

    // split into chunks and sort each chunk
    for ( i=0; i<size/Q; ++i )
    {
        Sort( a+i*Q, Q );
    }

    int m[ size/Q ];
    for ( i=0; i<size/Q; ++i )
    {
        m[i] = a[ i*Q + Q/2 ]; // median of each chunk
    }

    mm = Sel( size/Q/2, m, size/Q ); // median of medians

    // mark elements
    int marks[ size ];
    int countL=0;
    int countB=0;
    int countE=0; // countE = size - countL - countB
    for ( i=0; i<size; ++i )
    {
        if    ( a[i] < mm ) { ++countL; marks[ i ] = L; } // less
        elsif ( a[i] > mm ) { ++countB; marks[ i ] = B; } // bigger
        else                { ++countE; marks[ i ] = E; } // equal
    }

    if ( k < countL ) {
        int a2[ countL ];
        int j=0;
        for ( i=0; i<size; ++i ) { // copy smaller elements
            if ( marks[i] == L ) {
                a2[ j++ ] = a[i];
            }
        }
        Sel( k, a2, countL );
    } elsif ( k > countL+countE ) {
        int a2[ countB ];
        int j=0;
        for ( i=0; i<size; ++i ) { // copy bigger elements
            if ( marks[i] == B ) {
                a2[ j++ ] = a[i];
            }
        }
        Sel( k-countL-countE, a2, countB );
    } else {
        return mm; // all equal elements are the same
    }
}