 # [LeetCode] 150. Evaluate Reverse Polish Notation

## Problem

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

``````class Solution {
public int evalRPN(String[] tokens) {
Deque<Integer> stack = new ArrayDeque<>();
for (String token: tokens) {
if (token.equals("+")) {
stack.push(stack.pop()+stack.pop());
} else if (token.equals("-")) {
stack.push(-stack.pop()+stack.pop());
} else if (token.equals("*")) {
stack.push(stack.pop()*stack.pop());
} else if (token.equals("/")) {
int divisor = stack.pop();
int dividend = stack.pop();
stack.push(dividend/divisor);
} else {
stack.push(Integer.parseInt(token));
}
}
return stack.pop();
}
}``````