18_Reverse_Polish_Notation.java

// 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
 












// General approach and also optimal approach too

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> st=new Stack<>();
        for(String token:tokens){
            switch(token){
                case "+":
                    st.push(st.pop()+st.pop());
                    break;
                case "-":
                    int b=st.pop();
                    int a=st.pop();
                    st.push(a-b);
                    break;
                case "*":
                    st.push(st.pop()*st.pop());
                    break;
                case "/":
                    int div = st.pop();
                    int dvd = st.pop();
                    st.push(dvd / div);
                    break;
                default:
                    st.push(Integer.parseInt(token));
            }
        }
        return st.pop();
    }
}