Evaluate Reverse Polish Notation
Created: March 12, 2020 by [lek-tin]
Last updated: March 12, 2020
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
Valid operators are +, -, *, /. Each operand may be an integer or another expression.
Note
- Division between two integers should truncate toward zero.
- The given RPN expression is always valid. That means the expression would always evaluate to a result and there won’t be any divide by zero operation.
Example 1
Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9
Example 2
Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6
Example 3
Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation:
((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22
Solution
class Solution:
def evalRPN(self, tokens: List[str]) -> int:
if not tokens:
return None
stack = []
operators = set(["+", "-", "*", "/"])
N = len(tokens)
curr = 0
while curr < N:
while curr < N and tokens[curr] not in operators:
stack.append(tokens[curr])
curr += 1
while curr < N and tokens[curr] in operators:
op1 = int(stack.pop())
op2 = int(stack.pop())
temp = 0
if tokens[curr] == "+":
temp = op2 + op1
elif tokens[curr] == "-":
temp = op2 - op1
elif tokens[curr] == "*":
temp = op2 * op1
elif tokens[curr] == "/":
temp = int(op2 / op1)
stack.append(temp)
curr += 1
return stack.pop()