# 150. Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

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

Note:

  • Division between two integers should truncate toward zero. *The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: ["10", "6", "9", "3", "+", "-11", "*", "/", "*", "17", "+", "5", "+"]
Output: 22
Explanation: 
  ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

# Solution

Approach 1: Stack.

# Code (Python)

Approach 1:

# Code (C++)

Approach 1:

class Solution {
private:
    int getNum(string s) {
        int head = 0;
        int sign = 1;
        if (s[0] == '+' || s[0] == '-')
        {
            if (s[0] == '-')
                sign = -1;
            head++;
        }
        int num = 0;
        for (int i = head; i < s.size(); ++i)
        {
            num = num * 10 + (s[i] - '0');
        }
        return sign * num;
    }
    int doCalculation(int a, int b, char op) {
        switch (op)
        {
            case '+' : return a + b;
            case '-' : return a - b;
            case '*' : return a * b;
            case '/' : return a / b;
        }
        return 0;
    }
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> st;
        for (int i = 0; i < tokens.size(); ++i)
        {
            string s = tokens[i];
            if (s == "+" || s == "-" || s == "*" || s == "/")
            {
                int num2 = st.top();
                st.pop();
                int num1 = st.top();
                st.pop();
                st.push(doCalculation(num1, num2, s[0]));
            }
            else
            {
//                st.push(getNum(s));
//                or
                st.push(atoi(s.c_str()));
            }
        }
        return st.top();
    }
};