Evaluate Reverse Polish Notation

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.

Some examples:

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9

["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

原题链接: https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/

题目大意: 逆波兰表达式

思路: 利用栈实现, 注意+, -单目运算符; 另外目前leetcode使用的python为2.7.5, 除法不是精确的除法, 两个整数相除是地板除, 浮点数除是精确的除法; 注意负数的判断, 使用isdigit是判断不了的;

class Solution:
    # @param tokens, a list of string
    # @return an integer
    def evalRPN(self, tokens):
        stack = list()

        for token in tokens:
            try:
                tmp = int(token)
            except ValueError: #operator
                if token == '+': #may be a unary operator
                    if len(stack) > 1:
                        o1 = stack.pop()
                        o2 = stack.pop()

                        stack.append(o2 + o1)
                elif token == '-':
                    if len(stack) > 1:
                        o1 = stack.pop()
                        o2 = stack.pop()

                        stack.append(o2 - o1)
                    else:
                        o1 = stack.pop()
                        stack.append(-o1)

                elif token == '*':
                    o1 = stack.pop()
                    o2 = stack.pop()

                    stack.append(o2 * o1)
                else:
                    o1 = stack.pop()
                    o2 = stack.pop()

                    stack.append(int(float(o2) / o1))

            else: #number
                stack.append(tmp)

            
        return stack[0]