There is a binary tree stored in an array as:

$T[\,] = \{+, a, *, null, null, -, d, null, null, null, null, b, c\}$

in which $T[i]$ is the parent of $T[2i+1]$ and $T[2i+2]$, where $i$ is an index and null means there is no element in the slot.