Just to supplement the other answer, I think the “position of the leaf in the tree” means that leftmost “leaf index” is 0 and the rightmost is ( 2^height)-1 (if height does not include root).
┏━━━┓
root: ╭─────────────┨ r ┠─────────────╮
│ ┗━━━┛ │
┏━┷━┓ ┏━┷━┓
lyr2: ╭─────┨ x ┠─────╮ ╭─────┨ x ┠─────╮
│ ┗━━━┛ │ │ ┗━━━┛ │
┏━┷━┓ ┏━┷━┓ ┏━┷━┓ ┏━┷━┓
lyr1: ╭─┨ x ┠─╮ ╭─┨ x ┠─╮ ╭─┨ x ┠─╮ ╭─┨ x ┠─╮
│ ┗━━━┛ │ │ ┗━━━┛ │ │ ┗━━━┛ │ │ ┗━━━┛ │
┏━┷━┓ ┏━┷━┓ ┏━┷━┓ ┏━┷━┓ ┏━┷━┓ ┏━┷━┓ ┏━┷━┓ ┏━┷━┓
lry0: ┃ 0 ┃ ┃ 1 ┃ ┃ 2 ┃ ┃ 3 ┃ ┃ 4 ┃ ┃ 5 ┃ ┃ 6 ┃ ┃ 7 ┃
┗━━━┛ ┗━━━┛ ┗━━━┛ ┗━━━┛ ┗━━━┛ ┗━━━┛ ┗━━━┛ ┗━━━┛
height = 3
num_leaves = (2^height) = 2^3 = 8