卖萌的弱渣

I am stupid, I am hungry.

Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, * , /. Each operand may be an integer or another expression.

Examples

1
2

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

Solution

  • Use Stack
(Evaluate-Reverse-Polish-Notation.py) download
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

class Solution:
    # @param tokens, a list of string
    # @return an integer
    def evalRPN(self, tokens):
        stack = []
        for i in tokens:
            if i not in ('+', '-', '*', '/'):
                stack.append(int(i))
            else:
                # be careful of the order of op1 and op2
                op2 = stack.pop()
                op1 = stack.pop()
                if i == '+': stack.append(op1 + op2)
                elif i == '-': stack.append(op1 - op2)
                elif i == '*': stack.append(op1 * op2)
                else: stack.append(int(op1 * 1.0 / op2))
        return stack[0]