# Return first duplicate

This is a problem from Codesignal.

The guaranteed constraints are 1 <= a.length <= 10^5, and 1 <= a[i] <= a.length.

You have to return the first duplicate encoutered in a, or -1 if no duplicate is found.

def firstDuplicate(a):

b =  * 100001
for num in a:
if b[num] == 1:
return num
else:
b[num] = 1
return -1


Is there a better way to do this? Perhaps faster or using less memory? The unused 0 index in the b array also doesn't feel very clean.

• you can use the 0th index with num - 1, also if b[num]: return num is sufficient. Oct 13, 2020 at 23:40
• To save some space you can simply do b =  * (len(a) + 1). You know the elements are bound by 1 <= a[i] <= a.length so why blindly initialize to the maximum (10^5)... Doing num-1 adds one more operation per iteration and slows down - it is better to just have one slot not used... Oct 14, 2020 at 11:12
• The list -- which is essentially one implementation of a set -- could be filled with False and True directly and if b[num] == 1 could be replaced with if b[num].
– GZ0
Oct 14, 2020 at 11:55
• Serious question: What does Codesignal really want --- creativity? speed? footprint? I don't want to distract into the standard discussions of "Interview Questions," but maybe knowing the intent of the original question can better guide the answers. Oct 15, 2020 at 12:09
• @CarlWitthoft, they don't specify any criteria for optimization (perhaps unfortunately) besides that the solution has to run under 4 seconds, which is effectively the same as no constraint at all. I'm just using those websites to get better at algorithms. I try to optimize for speed and then for memory, knowing there's usually a tradeoff. Then come here for advice on how to do things better. I think for me the main appeal of those websites is the addictive quality that keeps me going all day on this stuff. Oct 15, 2020 at 18:40

# Memory

Your list implementation uses (depending on your architecture) 8 bytes per list element.

>>> import sys
>>> b = [False] * 100001
>>> sys.getsizeof(b)
800064


Note: This is just the memory of the list structure itself. In general, the contents of the list will use additional memory. In the "original" version, this would be pointers to two integer constants (0 and 1); in the "original_improved" version, it would be storing pointers to the two boolean singletons (False and True). Since we're only storing references to two objects in each case, this additional memory is insignificant, so we'll ignore it.

800kB of memory is not a huge amount, but to be polite, we can reduce it:

import array

def aneufeld_array(a):
b = array.array('b', (0,)) * (len(a) + 1)

for num in a:
if b[num]:
return num
b[num] = 1

return -1


Update: bytearray is a better choice than array.array('b', ...)!

def aneufeld_bytearray(a):
b = bytearray(len(a) + 1)

for num in a:
if b[num]:
return num
b[num] = 1

return -1


The bytearray(size) creates a tightly packed of bytes. Unlike lists, which can store different kinds of things in each element of the list, the bytearray, as its name implies, will only store bytes.

With this new structure, we now use only 1 byte per flag, so around 100kB of memory:

>>> b = bytearray(100001)
>>> sys.getsizeof(b)
100058


Performance wise, this solution is close to the original speed, so we're not giving up any significant speed while reducing the memory load to around 12.5%.

We can still do better. Using an entire byte for a single flag is wasteful; we can squeeze 8 flags into each byte. The bitarray class does all of the heavy lifting for us:

import bitarray

def aneufeld_bitarray(a):
b = bitarray.bitarray(len(a) + 1)
b.setall(False)

for num in a:
if b[num]:
return num
b[num] = True

return -1


This gets us down to 12.5kB for the bit-array of flags. Unfortunately, this additional memory optimization comes with an additional speed hit, due to the bit packing and unpacking. The performance is still better than "Sriv_improved" performance, and we're using only 1/64th of the original memory requirement.

Timing, on my machine:

 4.94 ms   4.62 ms   4.55 ms  original
3.89 ms   3.85 ms   3.84 ms  original_improved
20.05 ms  20.03 ms  19.78 ms  hjpotter92
9.59 ms   9.69 ms   9.75 ms  Reinderien
8.60 ms   8.68 ms   8.75 ms  Reinderien_improved
19.69 ms  19.69 ms  19.40 ms  Sriv
13.92 ms  13.99 ms  13.98 ms  Sriv_improved
6.84 ms   6.84 ms   6.86 ms  aneufeld_array
4.76 ms   4.80 ms   4.77 ms  aneufeld_bytearray
12.71 ms  12.65 ms  12.57 ms  aneufeld_bitarray


• @superbrain My bad - clearly I needed sleep. Fixed it, ... and then completely replaced it bytearray, which is far superior. Oct 15, 2020 at 15:32
• Yeah, using repeat is better in that sense, although it still costs more space (because the array overallocates) than array.array('b', (0,)) * (len(a) + 1), which is also shorter and about 2000 (!) times faster (for the worst case length). Oct 15, 2020 at 16:00
• @superbrain Clearly, Python needs an array.array(type, length) constructor, based on all the hoops we're going through to allocate an array of the desired size. Oct 15, 2020 at 16:08
• @superbrain Updated my times using your array multiplication suggestion. Nice. Oct 15, 2020 at 16:16
• Ha! Didn't think it would be noticeable in the solution time. I had only tested repeat-vs-multiply in isolation and repeat took about 0.006 seconds, which I thought was irrelevant in the overall solution time, but I forgot that I print the solution times in milliseconds :-) Oct 15, 2020 at 16:42

I'll propose an alternate implementation, because dictionaries are good at storing key-value pairs and you don't care about the value:

def first_duplicate(given_list):
seen = set()
for value in given_list:
if value in seen:
return value
return -1


A set will buy you basically the same behaviour as the "keys of a dictionary" for these purposes.

• It's nice and I usually would've written it like this as well, but given that the OP also asked "Perhaps faster or using less memory", I find it worth mentioning that in a worst case like list(range(1, 10**5+1)), this not only takes twice as much time as the original but also five times as much memory. (But I guess neither the 17 upvotes nor the OP's acceptance are concerned about that :-) Oct 15, 2020 at 14:28
• @superbrain Certainly; it's not the fastest, but I would venture to say that it's the "most canonical", if there is such a thing. Oct 15, 2020 at 14:35
• Yes, I certainly agree with that. Like I said I usually would've written it like this as well (probably have multiple times, even with the same name seen). Oct 15, 2020 at 14:38
• This approach applies to arbitrary lists. The OP's approach applies primarily to positive integer lists whose maximum is limited.
– GZ0
Oct 15, 2020 at 15:14
• @GZ0 That's how it should be advertised :-) Not quite arbitrary, though. Needs hashable elements. Oct 15, 2020 at 17:30

Benchmark and slightly improved versions of some solutions.

Congratulations, in the worst case (a = list(range(1, 10**5 + 1))) your original solution is about 2-4.5 times faster than the solutions in the previous answers:

 5.45 ms   5.46 ms   5.43 ms  original
4.58 ms   4.57 ms   4.57 ms  original_improved
25.10 ms  25.59 ms  25.27 ms  hjpotter92
11.59 ms  11.69 ms  11.68 ms  Reinderien
10.33 ms  10.47 ms  10.45 ms  Reinderien_improved
23.16 ms  23.07 ms  23.02 ms  Sriv
17.00 ms  16.97 ms  16.94 ms  Sriv_improved


Done with Python 3.9.0 64-bit on Windows 10 64-bit.

original_improved is yours but faster by not doing  == 1 and by using False instead of 0, as that's fastest to recognize as false. And for smaller input lists it takes less space as it makes b smaller accordingly.

Code:

from timeit import timeit
from collections import defaultdict

def original(a):
b =  * 100001
for num in a:
if b[num] == 1:
return num
else:
b[num] = 1
return -1

def original_improved(a):
b = [False] * (len(a) + 1)
for num in a:
if b[num]:
return num
b[num] = True
return -1

def hjpotter92(given_list):
seen = defaultdict(bool)
for value in given_list:
if seen[value]:
return value
seen[value] = True
return -1

def Reinderien(given_list):
seen = set()
for value in given_list:
if value in seen:
return value
return -1

def Reinderien_improved(given_list):
seen = set()
for value in given_list:
if value in seen:
return value
return -1

def Sriv(a):
for i in a:
if a[abs(i) - 1] > 0:
a[abs(i) - 1] *= -1
else:
return abs(i)
else:
return -1

def Sriv_improved(a):
for i in map(abs, a):
if a[i - 1] > 0:
a[i - 1] *= -1
else:
return i
else:
return -1

funcs = original, original_improved, hjpotter92, Reinderien, Reinderien_improved, Sriv, Sriv_improved

a = list(range(1, 10**5+1))

tss = [[] for _ in funcs]
for _ in range(4):
for func, ts in zip(funcs, tss):
t = min(timeit(lambda: func(copy), number=1)
for copy in (a.copy() for _ in range(50)))
ts.append(t)
for func, ts in zip(funcs, tss):
print(*('%5.2f ms ' % (t * 1000) for t in ts[1:]), func.__name__)
print()

• Comments are not for extended discussion; this conversation has been moved to chat. Oct 16, 2020 at 1:12

Perhaps, using a dictionary to keep account of already seen values?

from collections import defaultdict

def first_duplicate(given_list):
seen = defaultdict(bool)
for value in given_list:
if seen[value]:
return value
seen[value] = True
return -1


1. Function name should be lower_snake_case.
2. defaultdict initialises with default value as False. You can pass None instead of bool
3. Use better/descriptive names of variables.
4. Since if clause is returning a value, using the else clause is not needed.
• Thank you for the advice, I didn't know about defaultdict(). Out of curiosity, why is the lower_snake_case better practice? I actually used to do that and switched to camel because that's what they use on that website. Oct 14, 2020 at 1:55
• Because of PEP8. Oct 14, 2020 at 3:18
• For a worst case like list(range(1, 10**5+1)) this not only takes about 4.7 as much time as the original (see my answer) but also about 6.6 times as much memory as the original (and even 7.9 times as much in 32-bit Python). Oct 15, 2020 at 14:35

Note that all the elements are positive, and the values are not greater than the length.
There is a very clever method to find the solution in these cases.

The idea is to mark the values by turning a[value] negative.
If a duplicate exists, it will encounter a[duplicate] as negative.

Here's the implementation:

for i in a:
if a[abs(i) - 1] > 0:
a[abs(i) - 1] *= -1

else:
print(abs(i))
break

else:
print(-1)


Make sure to turn the values to 0-based indexing though!

This approach is O(N) time complexity and O(1) extra space complexity.

• It's not O(1) extra space complexity. The negative integer objects cost space. Oct 14, 2020 at 8:53
• Each single int, as long as it's bounded by a constant as it is here, takes constant space. But you don't just have a single one here. If I give you let's say a = list(range(1, N//2+1)) * 2, then you'll create new int objects for the first N/2 elements, which costs Θ(N) extra space. Not O(1). Oct 14, 2020 at 9:12
• For example for a = list(range(1, 314159)) * 2, the set solution builds a set of those numbers that takes 8.4 MB extra space, while you build the negations of those numbers and take 8.8 MB extra space: demo. That's with 64-bit Python. On 32-bit Python, it's 4.2 MB for the set and 5.0 MB for your numbers. Oct 14, 2020 at 19:11
• Well, in languages like C++ or Java, with primitive types like their int, you'd overwrite the original values with their negations indeed without using extra space. And if I weren't so mean to use a = list(range(1, N//2+1)) * 2 but instead used a = list(range(1, N+1)), then you might also take only O(1) extra space. Because you'd create the new number objects but the original number objects would be deleted since there'd be no references to them anymore. Then the new objects could occupy the memory where the originals were. It's just that my second half references the same objects, ... Oct 14, 2020 at 19:31
• Oops, check the demo again. 314159 is larger than allowed. 19661 is allowed and leads to 524 kB for the set and 550 kB for your negatives. That's btw the largest allowed one where set "wins" in this regard. At 19662, the set resizes to 2.1 MB. At the limit, i.e., with a = list(range(1, 50_001)) * 2, the set is still at 2.1 MB and your negatives reach 1.4 MB. So at that limit you do take less extra space than the set. Just not O(1) :-) Oct 14, 2020 at 19:41

Using list comprehension, make a list of the numbers occurring more than once

i for i in a if a.count(i) > 1


Return the first match or -1 if no match was found

next((i for i in a if a.count(i) > 1), -1)

• You totally live up to your name :-) Oct 14, 2020 at 12:31
• @superbrain How much slower? :) Oct 14, 2020 at 12:51
• In my answer's benchmark it would take about 17000 times as long as the original solution. Also: > 2? Oct 14, 2020 at 12:58
• Edited the 2. Of course I didn't try running it. I'm a bit disappointed the next doesn't prevent it from going through the whole list Oct 14, 2020 at 13:07