150. Evaluate Reverse Polish Notation
- Total Accepted: 76968
- Total Submissions: 304324
- Difficulty: Medium
- Contributors: Admin
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
Some examples:
["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9 ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6
Code
class Solution(object):
def evalRPN(self, tokens):
"""
:type tokens: List[str]
:rtype: int
"""
stk = []
for s in tokens:
try:
# test whether s is a number
int(s)
stk.append(s)
except:
a = stk.pop()
b = stk.pop()
stk.append(str(int(eval(str(float(b)) + s + a))))
return int(stk[-1])
Idea
Use stack. Every time you see an operator, pop the last element in the stack.