LC150. Evaluate Reverse Polish Notation

LC150

Discussion

A classical problem can be solved by using stack

Code

class Solution {
public:
    stack<int> stk;
    void eval(string op) {
        auto b = stk.top(); stk.pop();
        auto a = stk.top(); stk.pop();
        if (op == "+") stk.push(a + b);
        else if (op == "-") stk.push(a - b);
        else if (op == "*") stk.push( a * b);
        else stk.push(a / b);
        return;
    }
    int evalRPN(vector<string>& tokens) {
        unordered_set<string> op{"+", "-", "*", "/"};
        for (auto & c : tokens) {
            if (op.count(c)) eval(c);
            else stk.push(stoi(c));
        }
        return stk.top();
    }
};

Or

class Solution {
public:
    stack<int> stack;
    int evaluate(string operation){
        auto b = stack.top();stack.pop();
        auto a = stack.top();stack.pop();
        if (operation == "+") return a + b;
        if (operation == "-") return a - b;
        if (operation == "*") return a * b;
        if (operation == "/") return a / b;
        return 0;
    }
    int evalRPN(vector<string>& tokens) {
        unordered_set<string> operations = {"+", "-", "*", "/"};
        for (auto &c : tokens) {
            if (operations.count(c)) stack.push(evaluate(c));
            else stack.push(stoi(c));
        }
        return stack.top();
    }
};