void Heap::siftDown(int index) {
    int left = 2 * index + 1;
    int right = 2 * index + 2;
    int largest = index;
    if( left < length && buffer[left] > buffer[index])
        largest = left;
    if( right < length && buffer[right] > buffer[index])
        largest = right;
    if(largest != index ) {
        std::swap(buffer[index], buffer[largest]);
        siftDown( largest );
    }
}

void Heap::siftUp( int index ) {
    while( index > 0 ) {
        int parent = ( index - 1 ) / 2;
        if(buffer[index] <= buffer[parent])
            return;
        std::swap(buffer[index], buffer[parent] );
        index = parent;
    }
}