Implement a calculator that evaluates expressions in Reverse Polish Notation (RPN), also known as Postfix notation.

In RPN, operators follow their operands. For example, "3 4 +" means "3 + 4 = 7".

The input will be a string containing numbers and operators separated by spaces. The operators are +, -, *, and /. Division should perform integer division (truncate towards zero).

Space-separated digits e.g. treat "54" as one number in "54 +"

Examples:

Input: "3 4 +" Output: 7 (3 + 4 = 7) Input: "5 3 -" Output: 2 (5 - 3 = 2) Input: "3 5 * 2 +" Output: 17 ((3 * 5) + 2 = 17) Input: "7 2 / 3 +" Output: 6 ((7 / 2) + 3 = 6, integer division)
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