Evaluate Reverse Polish Notation

逆波兰表达式求值

最新推荐文章于 2022-03-25 16:52:08 发布

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本文介绍了一种通过逆波兰表达式（后缀表达式）计算算术表达式的算法实现。该方法使用列表存储表达式中的操作数和运算符，并通过遍历列表执行相应的数学运算得到最终结果。

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

class Solution:
    # @param tokens, a list of string
    # @return an integer
    def evalRPN(self, tokens):
        
        new_tokens = []
        for dummy_index in range(len(tokens)):
            new_tokens.append(tokens[dummy_index])
        
        result = 0
        
        if len(new_tokens) == 1:
            result = new_tokens[0]
            return int(result)
        
        label = True
        while label:
            for dummy_index in range(len(new_tokens)):
                if (new_tokens[dummy_index] == '+') or \
                   (new_tokens[dummy_index] == '-') or \
                   (new_tokens[dummy_index] == '*') or \
                   (new_tokens[dummy_index] == '/'):

                    if new_tokens[dummy_index] == '+':
                        result = int(new_tokens[dummy_index-2]) + int(new_tokens[dummy_index-1])
                    elif new_tokens[dummy_index] == '-':
                        result = int(new_tokens[dummy_index-2]) - int(new_tokens[dummy_index-1])
                    elif new_tokens[dummy_index] == '*':
                        result = int(new_tokens[dummy_index-2]) * int(new_tokens[dummy_index-1])
                    elif new_tokens[dummy_index] == '/':
                        
                        result = abs(int(new_tokens[dummy_index-2])) / abs((int(new_tokens[dummy_index-1])))
                        if (int(new_tokens[dummy_index-2]) > 0 and int(new_tokens[dummy_index-1]) < 0) or \
                           (int(new_tokens[dummy_index-2]) < 0 and int(new_tokens[dummy_index-1]) > 0):
                               result *= -1

                    new_tokens[dummy_index] = str(result)
                    del new_tokens[dummy_index - 1]
                    del new_tokens[dummy_index - 2]
                    
                    if len(new_tokens) == 1:
                        label = False
                    break
                
        return int(result)