[LeetCode 解題報告]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

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(atoi(tokens[i].c_str()));
            else {
                int m = s.top();
                s.pop();
                int n = s.top();
                s.pop();
                
                if (tokens[i] == "+")
                    s.push(m+n);
                if (tokens[i] == "-")
                    s.push(n-m);
                if (tokens[i] == "*")
                    s.push(m*n);
                if (tokens[i] == "/")
                    s.push(n/m);
            }
        }
        return s.top();
    }
};