Updated quicksort

prakhar1989 · prakhar1989 · commit c1e396ff45e2 · 2012-12-05T00:47:05.000+05:30
diff --git a/quicksort.py b/quicksort.py
@@ -0,0 +1,40 @@
+from random import randint
+def qsort(a, start, end):
+    """ quicksort in O(nlogn) and no extra
+    memory. In place implementation
+    >>> from random import sample
+    >>> rand_list = [sample(range(100), 10) for j in range(10)]
+    >>> sortedresult = [sorted(r) for r in rand_list]
+    >>> for r in rand_list: qsort(r, 0, len(r)-1)
+    >>> result = [sortedresult[i] == rand_list[i] for i in range(len(rand_list))]
+    >>> print sum(result)
+    10
+    """
+    if start < end:
+        p = choosepivot(start, end)
+        if p != start:
+            a[p], a[start] = a[start], a[p]
+        equal = partition(a, start, end)
+        qsort(a, start, equal-1)
+        qsort(a, equal+1, end)
+
+def partition(a, l, r):
+    """ partition array with pivot at a[0]
+    in the array a[l...r] that returns the
+    index of pivot element
+    """
+    pivot, i = a[l], l+1
+    for j in range(l+1, r+1):
+        if a[j] <= pivot:
+            a[i],a[j] = a[j],a[i]
+            i += 1
+    # swap pivot to its correct place
+    a[l], a[i-1] = a[i-1], a[l]
+    return i-1
+
+def choosepivot(s, e):
+    return randint(s,e)
+
+if __name__ == "__main__":
+    import doctest
+    doctest.testmod(verbose=True)
