Added fibonacci.py with fib(n) and main

fibonacci.py

@@ -0,0 +1,19 @@
+"""Fibonacci utilities."""
+
+def fibonacci(n):
+    """Return the nth Fibonacci number.
+
+    The sequence is defined by F(0) = 0, F(1) = 1 and F(n) = F(n-1) + F(n-2) for n >= 2.
+    This implementation uses an iterative approach with O(n) time and O(1) space.
+    """
+    if n < 0:
+        raise ValueError("n must be a non-negative integer")
+    a, b = 0, 1
+    for _ in range(n):
+        a, b = b, a + b
+    return a
+
+if __name__ == "__main__":
+    # Print the first 10 Fibonacci numbers: F(0) through F(9)
+    line = " ".join(str(fibonacci(i)) for i in range(10))
+    print(line)