4 ,5, 4, 2, ^, +, *, 2, 2, ^, 9, 3, /, *, -
→ 4, 5, 4^2, +, *, 2^2, 9/3, *, -
→ 4, 5, 16, +, *, 4, 3, *, -
→ 4, 5+16, *, 3*4, -
→ 4, 21, *, 12, -
→ 4*21, 12, -
→ 84-12
→ 72
Pseeduo code:
while tokens remain:
read the next token.
if the token is an operator:
pop the top two elements of the stack.
perform the operation on the elements.
push the result of the operation onto the stack.
else:
push the token (which must be a number)onto the stack