### MasterChief410's blog

By MasterChief410, history, 5 months ago, I noticed that many people were confused by the mathematics used in the soltuion of the editorial of this Div2.C question and decided to share with you my easy method for solving it. I use the property of distributivity of the lcm fucnction over gcd to simplify the solution. For three integers $a, b, c$ we have

• $\gcd(lcm(a, b), lcm(a, c)) = lcm(a, \gcd(b, c))$
• $lcm(\gcd(a, b), \gcd(a, c)) = \gcd(a, lcm(b, c))$

Hence for an array of integers,

• $\gcd(lcm(a_0, a_1), lcm(a_0, a_2) \dots lcm(a_0, a_n)) = lcm(a_0, gcd(a_1, a_2 \dots a_n))$
• $\gcd(lcm(a_1, a_2), lcm(a_1, a_3) \dots lcm(a_1, a_n)) = lcm(a_1, gcd(a_2, a_3 \dots a_n))$
• $\dots$ $and$ $so$ $on$

Therefore, for every element of the array we can precalculate the $\gcd$ of its next elements. Then we can take the lcm of that precalculated value with the element and store it in a new array. This way all possible pairs will be covered. The answer of the problem will be the $\gcd$ of these elements. Time complexity of the $\gcd(m, n)$ function is $\log_2v$ where $v=\max(m, n)$. Hence, time complexity of the solution will be $O(n\cdot\log_2maxval)$ where $maxval$ is the maximum of the array. Here is the solution:

#include <bits/stdc++.h>
using namespace std;

long long lcm(long long a, long long b) { return (a*b)/__gcd(a, b); }

int main()
{
long long n, ans=0;
cin>>n;
vector<long long> a(n), pregcd(n);

for(int i=0; i<n; i++)
cin>>a[i];

pregcd[n-2]=a[n-1];
for(int i=n-3; i>=0; i--)
pregcd[i]=__gcd(pregcd[i+1], a[i+1]);

for(int i=0; i<n-1; i++)
pregcd[i]=lcm(pregcd[i], a[i]);

for(int i=0; i<n-1; i++)
ans=__gcd(ans, pregcd[i]);
cout<<ans<<endl;
}


This is my first time writing a blog so suggestions are welcome! Edit: Changed the data type of variables from int to long long. #gcd, Comments (13)
 » 5 months ago, # | ← Rev. 2 →   Nice and simple solution, Great Work !!!
 » Nice solution
 » 5 months ago, # | ← Rev. 2 →   Please change the data type int to long long int! Or, it will give you WA! Nice solution...
•  » » Yeah! I forgot that. I'll edit the solution!
 » 5 months ago, # | ← Rev. 2 →   Nice solution.I think you don't know gcd(0,x) = x so every time you took first element then did gcd.
•  » » Yeah! Thanks, I'll edit the solution.
 » Great.This is very easy.Thanks a lot.
 » I don't know why I overkilled C like this:79898086. I calculated suffix gcds and then had to calculate lcm(a[i],suf[i+1]) for all valid i. I calculated this lcm by finding prime factorizations of a[i] and suf[i+1] for which I am kicking myself :(.
 » Great!I kwon how to solve it now.
 » 5 months ago, # | ← Rev. 2 →   Can't we just find gcd of n-1 elements one by one and one(pairwise adjacently) and later find the lcm of first element and the gcd result?
 » your editorial is awesome then actual everything from point to point
 » my superhero
•  » » XD