Add 2026.04.08 function stack example and homework

shared/notes-and-examples/2026.04.08/function_stack.py

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+# Simple function stack example: Fibonacci sequence
+# fibonacci(0) -> 0
+# fibonacci(1) -> 1
+# fibonacci(2) -> 1 (0 + 1)
+# fibonacci(3) -> 2 (1 + 1)
+# fibonacci(4) -> 3 (1 + 2)
+# fibonacci(5) -> 5 (2 + 3)
+# fibonacci(6) -> 8 (3 + 5)
+
+
+def fibonacci(n: int):
+    pass

shared/notes-and-examples/2026.04.08/homework.py

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+# Make a calculator using Reverse Polish Notation. Use recursion.
+# The Reverse Polish syntax completely removes the need for parentheses.
+#
+# Syntax examples:
+# "3 + 2" -> ["3", "2", "+"]
+# "3 * 4 + 2" -> ["3", "4", "*", "2", "+"]
+# "3 * (4 + 2)" -> ["3", "4", "2", "+", "*"]
+#
+# Example for how to evaluate ["3", "4", "2", "+", "*"]:
+# Start with "4 2 +" -> "6"
+# Then "3 6 *" -> "18"
+# Start with the "innermost" expression and work backwards (this is the recursion part)
+#
+# For help, email me@bchen.dev