Leetcode150. 逆波兰表达式求值

本体需要注意的问题：

1.数字有可能是负数
2.容器的类型是字符串，不是字符

示例：

输入：tokens = [“10”,“6”,“9”,“3”,"+","-11","","/","",“17”,"+",“5”,"+"]
输出：22
解释：该算式转化为常见的中缀算术表达式为：
((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

代码

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> st;
        int n = tokens.size();
        for (int i = 0; i < n; i++) {
            string& token = tokens[i];
            if (isNumber(token)) {
                st.push(atoi(token.c_str()));
            } else {
                int num2 = st.top();
                st.pop();
                int num1 = st.top();
                st.pop();
                switch (token[0]) {
                    case '+':
                        st.push(num1 + num2);
                        break;
                    case '-':
                        st.push(num1 - num2);
                        break;
                    case '*':
                        st.push(num1 * num2);
                        break;
                    case '/':
                        st.push(num1 / num2);
                        break;
                }
            }
        }
        return st.top();
    }

    bool isNumber(string& token) {
        return !(token == "+" || token == "-" || token == "*" || token == "/");
    }
};