Day6.cpp

// 150. Evaluate Reverse Polish Notation
// 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> s;
        int n=tokens.size();
        for(int i=0;i<n;i++){
            if(tokens[i]=="+" ||
               tokens[i]=="-" ||
               tokens[i]=="*" ||
               tokens[i]=="/" ){
                   int a=s.top();
                   s.pop();
                   int b=s.top();
                   s.pop();
                   if(tokens[i]=="+"){
                       s.push(a+b);
                   }
                   else if(tokens[i]=="-"){
                       s.push(b-a);
                   }
                   else if(tokens[i]=="*"){
                       s.push(a*b);
                   }
                   else{
                       s.push(b/a);
                   }
               }
            else{
                int a=stoi(tokens[i]);
                // cout<<a<<endl;
                s.push(a);
            }

        }
        return s.top();
    }
};