Solution/004

projects/004-flatten-a-list/python/main.py

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+'''
+Lists can contain other lists.   
+When one list occurs inside another the inner list is said to be nested inside the outer list. 
+
+Each of the inner lists nested within the outer list may also contain nested lists, 
+and those lists may contain additional nested lists to any depth. 
+
+For example, the following list includes elements that are nested at several different depths: 
+`[1, [2, 3], [4, [5, [6, 7]]], [[[8], 9], [10]]]`.
+
+
+Lists that contain multiple levels of nesting can be useful when describing complex relationships between values, 
+but such lists can also make performing some operations on those values difficult because the values 
+are nested at different levels. 
+
+Flattening a list is the process of converting a list that may contain multiple levels of nested lists 
+into a list that contains all of the same elements without any nesting. 
+For example, flattening the list from the previous paragraph results in `[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]`. 
+
+The following recursive algorithm can be used to flatten a list named data:
+
+    If data is empty then Return the empty list
+    If the first element in data is a list then
+        Set l1 to the result of flattening the first element in data
+        Set l2 to the result of flattening all of the elements in data, except the first
+        Return the concatenation of l1 and l2
+    If the first element in data is not a list then
+        Set l1 to a list containing only the first element in data
+        Set l2 to the result of flattening all of the elements in data, except the first Return the concatenation of l1 and l2
+
+Write a function that implements the recursive flattening algorithm described previously. 
+
+Your function will take one argument, which is the list to flatten, and it will return one result, which is the flattened list. 
+
+Include a main program that demonstrates that your function successfully flattens 
+the list shown earlier in this problem, as well as several others.
+'''
+
+def flattening(array, new_array=None):
+    if new_array is None:
+        new_array = []
+
+    if len(array) <= 0:
+        return new_array
+
+    elif isinstance(array[0], list):
+        flattening(array[0], new_array)
+
+    else:
+        new_array.append(array[0])
+
+    if len(array) > 1:
+        flattening(array[1:], new_array)
+
+    return new_array
+
+def main():
+    array_original = [1, [2, 3], [4, [5, [6, 7]]], [[[8], 9], [10]]]
+    flatten = flattening(array_original)
+    print(flatten)
+
+if __name__ == '__main__':
+    main()