Easy Operand Stack Stacks & Queues
In plain English: Calculate the result of a math expression written in postfix notation like '2 1 + 3 *'.
Postfix notation eliminates the need for parentheses and precedence — operands are always ready on the stack when an operator appears.
Prompt
Evaluate a postfix (reverse Polish notation) expression.
Try to write it from scratch before scrolling down.
Solution
def eval_postfix(tokens):
stack = []
ops = {'+': lambda a,b: a+b, '-': lambda a,b: a-b,
'*': lambda a,b: a*b, '/': lambda a,b: int(a/b)}
for t in tokens:
if t in ops:
b, a = stack.pop(), stack.pop() # pop order matters: b first, then a
stack.append(ops[t](a, b))
else:
stack.append(int(t))
return stack[0]
# Test: eval_postfix(["2","1","+","3","*"]) == 9