You are given an array of strings `tokens` that represents an arithmetic expression in **Reverse Polish Notation**.
Evaluate the expression and return an integer that represents the value of the expression.
**Note:** Valid operators are `+`, `-`, `*`, and `/`. Each operand may be an integer or another expression. Division truncates toward zero.
**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
```