Problem 21. Return the 3n+1 sequence for n

Created by Cody Team

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A Collatz sequence is the sequence where, for a given number n, the next number in the sequence is either n/2 if the number is even or 3n+1 if the number is odd. The sequence always terminates with 1.
So if
 n = 13
then
 c = [13    40    20    10     5    16     8     4     2     1]

Solve

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Last Solution submitted on Nov 06, 2025

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Cedric on 5 Apr 2013

Hep, Cody Team, could you remove my solution please?

Cedric on 8 Apr 2013

Thank you!

Mandeep Singh on 14 Jun 2013

nice one but consumes size dunno how to reduce it

jubin soni on 13 Nov 2016

a bit tricky but nice

Dude on 17 Feb 2017

I like this problem, because I learnt about the Collatz sequence story. Very interessting.

Anant on 23 Mar 2024

function c = collatz(n)
c(1) = n;
while (1>0)
if (n == 1)
c(end+1) = n;
break
elseif (mod(n,2) == 0)
c(end+1) = n;
n = n/2;
collatz(n);
else
c(end+1) = n;
n = 3*n + 1;
collatz(n);
end
end
c(1) = [];
end

This solution is correct as I ran the Test Cases myself. But the compiler timed out. What should I do ?

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Problem 21. Return the 3n+1 sequence for n

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Problem 21. Return the 3n+1 sequence for n

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