150. Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression inReverse Polish Notation.

Valid operators are+,-,*,/. Each operand may be an integer or another expression.

Some examples:

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

S: stack O(n)

    int evalRPN(vector<string>& tokens) {
        if (tokens.size() == 0) return 0;
        stack<int> nums;
        unordered_set<string> operators{"+", "-", "*", "/"};
        for (const string& s : tokens) {
            if (operators.find(s) == operators.end()) {
                nums.push(stoi(s));
            } else {
                int b = nums.top();
                nums.pop();
                int a = nums.top();
                nums.pop();
                if (s == "+") {
                    nums.push(a + b);
                } else if (s == "-") {
                    nums.push(a - b);
                } else if (s == "*") {
                    nums.push(a * b);
                } else if (s == "/") {
                    nums.push(a / b);
                }
            }
        }
        return nums.top();
    }