evaluate_reverse_polish_notation.py

classSolution:
defevalRPN(self, tokens: List[str]) ->int:

# main idea:
# The Reverse Polish Notation (RPN) is also known as 
# the 'postfix' notation, because each operator appears after its operands.
# Stack fits perfectly as it is a Last-In-First-Out (LIFO) data structure.

my_stack= []
operators= ['+', '-', '*', '/']

fortokenintokens:
iftokennotinoperators:
operand=int(token) # we need to change it to 'integer'
my_stack.append(operand) # push
else:
a=my_stack.pop()
b=my_stack.pop()
# be careful: need to use 'b-a' and 'b//a'
iftoken=='+':
c=b+a
my_stack.append(c)
eliftoken=='-':
c=b-a
my_stack.append(c)
eliftoken=='*':
c=b*a
my_stack.append(c) 
else:
# special case (becasue Python is different from C language)
ifb*a<0: 
c=-((-b) //a)
my_stack.append(c) 
else: 
c=b//a
my_stack.append(c) 

final_value=my_stack.pop() # pop the final value in the stack
returnfinal_value