Permutations solution was added.

permutations/permutations-serge.py

@@ -0,0 +1,27 @@
+# Backtracking solution
+# O(n * n!) time & space complexity
+
+
+def _recursive_permutations(permutation, sequence, picked, result):
+    if len(picked) == len(sequence):
+        result.append(permutation)
+        return
+    for i, elem in enumerate(sequence):
+        if i not in picked:
+            _recursive_permutations(permutation + [elem], sequence,
+                                    picked | {i}, result)
+
+
+def get_permutations(s):
+    result = []
+    _recursive_permutations([], s, set(), result)
+
+    return [''.join(permutation) for permutation in result]
+
+
+if __name__ == '__main__':
+    assert get_permutations('') == ['']
+    assert get_permutations('a') == ['a']
+    assert get_permutations('ab') == ['ab', 'ba']
+    assert get_permutations('abc') == ['abc', 'acb', 'bac', 'bca', 'cab', 'cba']
+    assert get_permutations('aaa') == ['aaa', 'aaa', 'aaa', 'aaa', 'aaa', 'aaa']