2、evaluate-reverse-polish-notation

题目描述

Evaluate the value of an arithmetic expression in Reverse
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

//判断字符串是否是数字，如果是就压栈，不是就弹出2个元素进行计算，将计算结果压栈

//最后栈顶元素即为所求

class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        stack<int> st;
        for (int i = ; i < tokens.size(); i++){
            if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {
                int temp = ;
                int a = st.top();
                st.pop();
                int b = st.top();
                st.pop();
                if (tokens[i] == "+") { temp = b + a; }
                else if (tokens[i] == "-") { temp = b - a; }
                else if (tokens[i] == "*") { temp = b * a; }
                else { temp = b / a; }
                st.push(temp);
            }
            else {
                st.push(stoi(tokens[i]));
            }
        }
        return st.top();
    }
};