Evaluate Reverse Polish 20 XP Medium

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Reverse Polish Notation (RPN) puts operators after their operands. No parentheses needed!

# Normal:   (2 + 1) * 3
# RPN:      2 1 + 3 *
# Step-by-step:
# Push 2 → [2]
# Push 1 → [2, 1]
# +     → pop 1,2 → push 3 → [3]
# Push 3 → [3, 3]
# *     → pop 3,3 → push 9 → [9]
# Answer: 9

Algorithm using a stack:

If token is a number, push it onto the stack
If token is an operator (+, -, *, /), pop two values, compute, push result
The order matters for subtraction and division: second_popped OP first_popped
Division truncates toward zero: use int(a/b) not a//b

This is how calculators and compilers actually evaluate expressions!

📋 Instructions

Implement `eval_rpn(tokens)` to evaluate a Reverse Polish Notation expression. Test with ["2","1","+","3","*"] and ["4","13","5","/","+"].

For each token: if it's a digit (or negative number), push int(token). If it's an operator, pop b then a, compute a OP b, and push the result. Remember: int(a/b) truncates toward zero.

🧪 Test Cases

Input

eval_rpn(["2","1","+","3","*"])

Expected

9

Test case 1

Input

eval_rpn(["4","13","5","/","+"])

Expected

6

Test case 2

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main.py

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Output

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