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Merge Sort in Python
#!/usr/bin/env python
"""
An implementation in python, inspired by the haskell version via
Cormen
(https://github.com/dpapathanasiou/algorithms-unlocked-haskell/blob/master/algorithms-for-sorting-and-searching/MergeSort.hs)
"""
def merge (a, b, c=[]):
if len(a) == 0:
return c + b
elif len(b) == 0:
return c + a
else:
ah = a[0]
bh = b[0]
if ah <= bh:
return merge (a[1:], b, c + [ah])
else:
return merge (a, b[1:], c + [bh])
def mergesort (a):
l = len(a)
if l < 2:
return a
else:
m = l // 2
return merge(mergesort(a[0:m]), mergesort(a[m:]))
"""
Test it, using two types of arrays:
1 - a numeric array
2 - a string (array of chars)
"""
narr = [10,2,5,3,1,6,7,4,2,3,4,8,9]
nexp = [1,2,2,3,3,4,4,5,6,7,8,9,10]
nres = mergesort(narr)
print 'sorting', narr, '->', nres
assert nres == nexp, 'expected ' + nexp + ' from sorting ' + narr + ' but got ' + nres
# using list() to keep the type system happy
sarr = list("the quick brown fox jumps over the lazy dog")
sexp = list(" abcdeeefghhijklmnoooopqrrsttuuvwxyz")
sres = mergesort(sarr)
print 'sorting "' + ''.join(sarr) + '" -> "' + ''.join(sres) + '"'
assert sres == sexp, 'expected "' + ''.join(sexp) + '" from sorting "' + ''.join(sarr) + '" but got "' + ''.join(sres) + '"'
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