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    <p>Hoare’s Quicksort algorithm chooses a random pivot. For reproducible results I'd suggest Scowen's modification which, among other things, chooses the middle item from the input. For this variant a sawtooth pattern with the pivot being the smallest appears to be the worst case input:</p> <pre><code>starting with { j i h g f a e d c b } compare 1 to 6 { (j) i h g f (a) e d c b } compare 6 to 10 { j i h g f (a) e d c (b) } compare 6 to 9 { j i h g f (a) e d (c) b } compare 6 to 8 { j i h g f (a) e (d) c b } compare 6 to 7 { j i h g f (a) (e) d c b } swap 1&lt;=&gt;6 { (a) i h g f (j) e d c b } compare 1 to 5 { (a) i h g (f) j e d c b } compare 1 to 4 { (a) i h (g) f j e d c b } compare 1 to 3 { (a) i (h) g f j e d c b } compare 1 to 2 { (a) (i) h g f j e d c b } compare 2 to 6 { a (i) h g f (j) e d c b } compare 3 to 6 { a i (h) g f (j) e d c b } compare 4 to 6 { a i h (g) f (j) e d c b } compare 5 to 6 { a i h g (f) (j) e d c b } compare and swap 6&lt;=&gt;10 { a i h g f (b) e d c (j) } compare 7 to 10 { a i h g f b (e) d c (j) } compare 8 to 10 { a i h g f b e (d) c (j) } compare 9 to 10 { a i h g f b e d (c) (j) } compare 2 to 6 { a (i) h g f (b) e d c j } compare 6 to 9 { a i h g f (b) e d (c) j } compare 6 to 8 { a i h g f (b) e (d) c j } compare 6 to 7 { a i h g f (b) (e) d c j } swap 2&lt;=&gt;6 { a (b) h g f (i) e d c j } compare 2 to 5 { a (b) h g (f) i e d c j } compare 2 to 4 { a (b) h (g) f i e d c j } compare 2 to 3 { a (b) (h) g f i e d c j } compare 3 to 6 { a b (h) g f (i) e d c j } compare 4 to 6 { a b h (g) f (i) e d c j } compare 5 to 6 { a b h g (f) (i) e d c j } compare and swap 6&lt;=&gt;9 { a b h g f (c) e d (i) j } compare 7 to 9 { a b h g f c (e) d (i) j } compare 8 to 9 { a b h g f c e (d) (i) j } compare 3 to 6 { a b (h) g f (c) e d i j } compare 6 to 8 { a b h g f (c) e (d) i j } compare 6 to 7 { a b h g f (c) (e) d i j } swap 3&lt;=&gt;6 { a b (c) g f (h) e d i j } compare 3 to 5 { a b (c) g (f) h e d i j } compare 3 to 4 { a b (c) (g) f h e d i j } compare 4 to 6 { a b c (g) f (h) e d i j } compare 5 to 6 { a b c g (f) (h) e d i j } compare and swap 6&lt;=&gt;8 { a b c g f (d) e (h) i j } compare 7 to 8 { a b c g f d (e) (h) i j } compare 4 to 6 { a b c (g) f (d) e h i j } compare 6 to 7 { a b c g f (d) (e) h i j } swap 4&lt;=&gt;6 { a b c (d) f (g) e h i j } compare 4 to 5 { a b c (d) (f) g e h i j } compare 5 to 6 { a b c d (f) (g) e h i j } compare and swap 6&lt;=&gt;7 { a b c d f (e) (g) h i j } compare and swap 5&lt;=&gt;6 { a b c d (e) (f) g h i j } </code></pre>
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    1. POObserving quadratic behavior with quicksort - O(n^2)
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    1. USHandcraftsman
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