evaluate-reverse-polish-notation.md

150. Evaluate Reverse Polish Notation

Medium

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You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

• The valid operators are '+', '-', '*', and '/'.
• Each operand may be an integer or another expression.
• The division between two integers always truncates toward zero.
• There will not be any division by zero.
• The input represents a valid arithmetic expression in a reverse polish notation.
• The answer and all the intermediate calculations can be represented in a 32-bit integer.

 

Example 1:

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

Example 2:

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

Example 3:

Input: tokens = ["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

 

Constraints:

• 1 <= tokens.length <= 104
• tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].

Solution

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> stk;
        int a,b;
        for(string s:tokens){
            if(s.size()>1||isdigit(s[0])){
                stk.push(stoi(s));
                continue;
            }
            a=stk.top();
            stk.pop();
            b=stk.top();
            stk.pop();
            switch(s[0]){
                case '*':
                    stk.push(b*a);
                    break;
                case '/':
                    stk.push(b/a);
                    break;
                case '+':
                    stk.push(b+a);
                    break;
                case '-':
                    stk.push(b-a);
                    break;
            }
        }
        return stk.top();
    }
};