leetcode_evaluate reverse polish notation

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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.
（计算逆波兰表达式）

Note:

  1. Division between two integers should truncate toward zero.
  2. 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. 栈

使用堆栈来计算逆波兰表达式。其中需要注意的是，题目中要求除法运算没有整除时要向 0 靠近，因此在两个异号的数相除时，应该 + 1 处理，或者转换为正数除法再加负号。具体实现过程如下：

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class (object):
    def evalRPN(self, tokens):
        """
        :type tokens: List[str]
        :rtype: int
        """
        stack = []
        
        for i in range(len(tokens)):
            if tokens[i] not in ['+', '-', '*', '/']:
                stack.append(int(tokens[i]))
            else:
                right = stack.pop()
                left = stack.pop()
                if tokens[i] == '+':
                    stack.append(left + right)
                elif tokens[i] == '-':
                    stack.append(left - right)
                elif tokens[i] == '*':
                    stack.append(left * right)
                else:
                    
                    if left * right < 0 and left % right != 0:
                        stack.append(left / right + 1)
                        # stack.append(-(abs(left) / abs(right)))
                    else:
                        stack.append(left / right)
        
        return stack[-1]