Given an array of strings tokens representing a valid Reverse Polish Notation (RPN) expression, evaluate it and return the result.

In Reverse Polish Notation, operators come after their operands. For example, the expression (2 + 1) * 3 in standard notation becomes 2 1 + 3 * in RPN.

The valid operators are +, -, *, and /. Division between integers truncates toward zero.

tokens: an array of strings, each representing either a number or an operator (+, -, *, /)

An integer representing the result of evaluating the RPN expression

Output: 9

This represents: (2 + 1) * 3

Step 1: 2 + 1 = 3

Step 2: 3 * 3 = 9

Result: 9

Example 2:

Input: tokens = ["4", "13", "5", "/", "+"]

Output: 6

This represents: 4 + (13 / 5)

Step 1: 13 / 5 = 2 (integer division)

Step 2: 4 + 2 = 6

Result: 6

Example 3:

Input: tokens = ["10", "6", "9", "3", "/", "-", "*", "17", "+", "5", "+"]

Output: 52

This represents: ((10 * (6 - (9 / 3))) + 17) + 5

Step 1: 9 / 3 = 3

Step 2: 6 - 3 = 3

Step 3: 10 * 3 = 30

Step 4: 30 + 17 = 47

Step 5: 47 + 5 = 52