add boilerplate for second edition

first_edition/ebook_src/builtin_structures/permutations.py

@@ -0,0 +1,70 @@
+#!/usr/bin/env python
+
+__author__ = "bt3"
+
+
+
+def perm(str1):
+    '''
+    >>> perm('123')
+    ['123', '132', '231', '213', '312', '321']
+    '''
+
+    if len(str1) < 2:
+        return str1
+
+    res = []
+    for i, c in enumerate(str1):
+        for cc in perm(str1[i+1:] + str1[:i]):
+            res.append(c + cc)
+    return res
+
+
+def perm2(str1):
+    '''
+    >>> perm2('123')
+    ['123', '132', '213', '231', '312', '321']
+    '''
+    from itertools import permutations
+    return [''.join(p) for p in permutations(str1)]
+
+
+def ispermutation(s1, s2):
+    '''
+    >>> ispermutation('231', '123')
+    True
+    >>> ispermutation('231', '153')
+    False
+    '''
+
+    from collections import Counter
+    aux = Counter()
+    for i in s1:
+        aux[i] += 1
+    for i in s2:
+        aux[i] -= 1
+    for v in aux.values():
+        if v != 0:
+            return False
+    return True
+
+
+
+
+def ispermutation2(s1, s2):
+    '''
+    >>> ispermutation2('231', '123')
+    True
+    >>> ispermutation2('231', '153')
+    False
+    '''
+    if sorted(s1) == sorted(s2):
+        return True
+    else:
+        return False
+
+
+
+if __name__ == '__main__':
+    import doctest
+    doctest.testmod()