[LeetCode] Evaluate Reverse Polish Notation | 逆波兰表达式求值

https://leetcode.com/problems/evaluate-reverse-polish-notation/?tab=Description

开一个栈，遇到数字压栈，遇到运算符就从栈中弹出两个元素做相应的计算然后再压回去。

Time complexity: O(n)
Space complexity: O(n)

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        if (tokens.empty()) return 0;
        if (tokens.size() == 1) return stoi(tokens[0]);
        
        stack<int> stk;
        int ret = 0;
        for (int i = 0; i < tokens.size(); ++i) {
            string& str = tokens[i];
            if (isop(str)) {
                int x = stk.top(); stk.pop();
                int y = stk.top(); stk.pop();
                if (str == "+") {
                    ret = y + x;
                } else if (str == "-") {
                    ret = y - x;
                } else if (str == "*") {
                    ret = y * x;
                } else {
                    ret = y / x;
                }
                stk.push(ret);
            } else {
                stk.push(stoi(str));
            }
        }
        
        return ret;
    }
private:
    bool isop(string& str) {
        return (str == "+" || str == "-" || str == "*" || str == "/");
    }
};