leetcode 150. Evaluate Reverse Polish Notation

Posted 2021-12-07 ymjyqsx

https://www.cnblogs.com/grandyang/p/4247718.html

逆波兰表达式就是把操作数放前面，把操作符后置的一种写法。用stack解决就好了。

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        if(tokens.empty())
            return 0;
        stack<int> s;
        for(int i = 0;i < tokens.size();i++){
            if(tokens[i] != "+" && tokens[i] != "-" && tokens[i] != "*" && tokens[i] != "/")
                s.push(stoi(tokens[i]));
            else{
                int num1 = s.top();
                s.pop();
                int num2 = s.top();
                s.pop();
                int tmp;
                if(tokens[i] == "+")
                    tmp = num1 + num2;
                else if(tokens[i] == "-")
                    tmp = num2 - num1;
                else if(tokens[i] == "*")
                    tmp = num1 * num2;
                else if(tokens[i] == "/")
                    tmp = num2 / num1;
                s.push(tmp);
            }
        }
        return s.top();
    }
};