输入字符串表达式S，实现一个基本计算器来计算，并返回计算的值

要实现一个基于字符串表达式的简单计算器，你可以采用递归解析器或者使用现有的库（如Python的`eval()`函数，但这可能会带来安全风险），这里我将提供一个基础的解析方法来处理加减乘除运算。这个示例仅适用于整数运算，并假设输入不会包含非法字符。

def calculate_expression(s):
    def apply_operator(operands, operator):
        if operator == '+':
            return operands[0] + operands[1]
        elif operator == '-':
            return operands[0] - operands[1]
        elif operator == '*':
            return operands[0] * operands[1]
        elif operator == '/':
            # 确保除数不为零
            if operands[1] != 0:
                return operands[0] / operands[1]
            else:
                raise ValueError("Division by zero is not allowed.")

    def parse_token(token, operators, operands):
        if token.isdigit():
            operands.append(int(token))
        elif token in operators:
            if not operands or not operands[-1].isdigit():
                raise ValueError("Invalid expression")
            operands.append(apply_operator(operands, token))
            operands.pop()  # 移除刚应用操作符后的数字

    # 分割输入字符串并处理每个部分
    tokens = s.split()
    if len(tokens) < 2:
        raise ValueError("Invalid expression (at least one operator needed)")

    operators = ['+', '-', '*', '/']
    operands = []
    for token in tokens:
        parse_token(token, operators, operands)

    # 最后一个元素应该是结果
    if not operands:
        raise ValueError("No result found after parsing")

    return operands[0]

# 示例
expression = "5 + 3 * 2"
result = calculate_expression(expression)
print(f"Result of {expression}: {result}")