Node A in position 1

Node B in position 2

Node C in position 3

...

Left child of Node in position K: in position 2K

Right child of Node in position K: in position 2K + 1

So D is in position 2*2, E in 2*2+1, F in 2*3, G in 2*3+1.

Tree with N nodes does not always occupy first N array positions: