evaluate_reverse_polish_notation.md

Location: https://leetcode.com/problems/evaluate-reverse-polish-notation/description/

Question

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

The valid operators are '+', '-', '*', and '/'. Each operand may be an integer or another expression. The division between two integers always truncates toward zero. There will not be any division by zero. The input represents a valid arithmetic expression in a reverse polish notation. The answer and all the intermediate calculations can be represented in a 32-bit integer.

Example

Example 1:

• Input: tokens = ["2","1","+","3","*"]
• Output: 9
• Explanation: ((2 + 1) * 3) = 9

Example 2:

• Input: tokens = ["4","13","5","/","+"]
• Output: 6
• Explanation: (4 + (13 / 5)) = 6

Example 3:

• Input: tokens = ["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

My solution (follow instructions of others)

class Solution(object):
    def evalRPN(self, tokens):
        """
        :type tokens: List[str]
        :rtype: int
        """
        st = []
        for t in tokens:
            if t not in ['+', '-', '*', '/']:
                st.append(int(t))
            else:
                r, l = st.pop(), st.pop()
                if t  == "+":
                    st.append(l+r)
                elif t == '-':
                    st.append(l-r)
                elif t == '*':
                    st.append(l*r)
                else:
                    st.append(int(float(l)/r))
        return st.pop(0)