Add recursive and iterative flatten examples for python-flatten-list (#760)

Co-authored-by: realpython-bot <noreply@realpython.com>

Author: realpython-bot

python-flatten-list/flatten_iterative.py

@@ -0,0 +1,15 @@
+def flatten(nested):
+    flat = []
+    stack = [iter(nested)]
+    while stack:
+        for item in stack[-1]:
+            if isinstance(item, list):
+                stack.append(iter(item))
+                break
+            flat.append(item)
+        else:
+            stack.pop()
+    return flat
+
+
+flatten([1, [2, [3, [4, [5, 6]]]], 7, [8, 9]])

python-flatten-list/flatten_recursive.py

@@ -0,0 +1,12 @@
+def flatten(nested):
+    flat = []
+    for item in nested:
+        if isinstance(item, list):
+            flat.extend(flatten(item))
+        else:
+            flat.append(item)
+    return flat
+
+
+nested = [1, [2, [3, [4, [5, 6]]]], 7, [8, 9]]
+flatten(nested)