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/*
Randomised quick sort implementation in C language.
In normal quick sort, pivot chosen to partition is either the first or the last element of the array.
This can take time O(n*n) to sort in the worst case.
Now in randomised quick sort, pivot is randomly chosen and then recursively sort the left and right sub-arrays.
The expected running time of the algorithm is O(nlog(n)).
*/
#include
<stdio.h>
#include
<stdlib.h>
#include
<time.h>
int
getBig
(
int
a
[],
int
i
,
int
right
,
int
pivot
)
{
for
(
int
k
=
i
;
k
<=
right
;
k
++
)
{
if
(
a
[
k
]
>
pivot
)
return
k
;
}
return
right
+
1
;
}
int
getSmall
(
int
a
[],
int
j
,
int
left
,
int
pivot
)
{
for
(
int
k
=
j
;
k
>=
left
;
k
--
)
{
if
(
a
[
k
]
<
pivot
)
return
k
;
}
return
-1
;
}
void
swap
(
int
*
a
,
int
*
b
)
{
int
t
=
*
a
;
*
a
=
*
b
;
*
b
=
t
;
}
void
random_quick
(
int
a
[],
int
left
,
int
right
)
{
if
(
left
>=
right
)
return
;
int
index
=
left
+
(
rand
()%(
right
-
left
)),
i
=
left
,
j
=
right
;
int
pivot_index
=
index
;
int
pivot
=
a
[
index
];
// storing index of element greater than pivot
i
=
getBig
(
a
,
i
,
right
,
pivot
);
// storing index of element smaller than pivot
j
=
getSmall
(
a
,
j
,
left
,
pivot
);
while
(
i
<=
j
)
{
swap
(
&
a
[
i
],
&
a
[
j
]);
i
=
getBig
(
a
,
i
,
right
,
pivot
);
j
=
getSmall
(
a
,
j
,
left
,
pivot
);
}
// after separating the smaller and greater elements, there are 3 cases possible
if
(
pivot_index
>
j
&&
pivot_index
>
i
)
{
// case 1. When the pivot element index is greater than both i and j
swap
(
&
a
[
i
],
&
a
[
pivot_index
]);
random_quick
(
a
,
left
,
i
-
1
);
random_quick
(
a
,
i
+
1
,
right
);
}
else
if
(
pivot_index
<
j
&&
pivot_index
<
i
)
{
// case 2. When the pivot element index is smaller than both i and j
swap
(
&
a
[
j
],
&
a
[
pivot_index
]);
random_quick
(
a
,
left
,
j
-
1
);
random_quick
(
a
,
j
+
1
,
right
);
}
else
{
// the pivot element is at its origin position.
random_quick
(
a
,
left
,
pivot_index
-
1
);
random_quick
(
a
,
pivot_index
+
1
,
right
);
}
}
int
main
()
{
srand
(
time
(
0
));
int
num
;
scanf
(
"%d"
,
&
num
);
int
arr
[
num
];
for
(
int
i
=
0
;
i
<
num
;
i
++
)
{
scanf
(
"%d"
,
&
arr
[
i
]);
}
random_quick
(
arr
,
0
,
num
-
1
);
for
(
int
i
=
0
;
i
<
num
;
i
++
)
{
printf
(
"%d "
,
arr
[
i
]);
}
printf
(
"\n"
);
}
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