# Find a number that divides maximum array elements

Given an array A[] of N non-negative integers. Find an Integer greater than 1, such that maximum array elements are divisible by it. In case of same answer print the smaller one.

Examples:

Input : A[] = { 2, 4, 5, 10, 8, 15, 16 };
Output : 2
Explanation: 2 divides [ 2, 4, 10, 8, 16] no other element divides greater than 5 numbers.

Input : A[] = { 2, 5, 10 }
Output : 2
Explanation: 2 divides [2, 10] and 5 divides [5, 10], but 2 is smaller.

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Naive Approach: Run a for loop upto maximum element of the array. Let it be K. Iterate the array and divide each element of the array by all numbers . Update the result according the maximum number of elements got divided by the element i.

Efficient Approach: We know that a number can be divisible only by elements which can be formed by their prime factors.
Thus we find the prime factors of all element of the array and store their frequency in the hash. Finally we return the element with maximum frequency among them.

You can use factorization-using-sieve to find prime factors in Log(n).

Below is the implementation of above approach:

## C++

 // CPP program to find a number that // divides maximum array elements    #include using namespace std;    #define MAXN 100001    // stores smallest prime factor for every number int spf[MAXN];    // Calculating SPF (Smallest Prime Factor) for every // number till MAXN. // Time Complexity : O(nloglogn) void sieve() {     spf[1] = 1;     for (int i = 2; i < MAXN; i++)            // marking smallest prime factor for every         // number to be itself.         spf[i] = i;        // separately marking spf for every even     // number as 2     for (int i = 4; i < MAXN; i += 2)         spf[i] = 2;        for (int i = 3; i * i < MAXN; i++) {         // checking if i is prime         if (spf[i] == i) {             // marking SPF for all numbers divisible by i             for (int j = i * i; j < MAXN; j += i)                    // marking spf[j] if it is not                 // previously marked                 if (spf[j] == j)                     spf[j] = i;         }     } }    // A O(log n) function returning primefactorization // by dividing by smallest prime factor at every step vector getFactorization(int x) {     vector ret;     while (x != 1) {         int temp = spf[x];         ret.push_back(temp);         while (x % temp == 0)             x = x / temp;     }     return ret; }    // Function to find a number that // divides maximum array elements int maxElement(int A[], int n) {     // precalculating Smallest Prime Factor     sieve();        // Hash to store frequency of each divisors     map m;        // Traverse the array and get spf of each element     for (int i = 0; i < n; ++i) {            // calling getFactorization function         vector p = getFactorization(A[i]);            for (int i = 0; i < p.size(); i++)             m[p[i]]++;     }        int cnt = 0, ans = 1e+7;        for (auto i : m) {         if (i.second >= cnt) {             cnt = i.second;             ans > i.first ? ans = i.first : ans = ans;         }     }        return ans; }    // Driver program int main() {     int A[] = { 2, 5, 10 };     int n = sizeof(A) / sizeof(A[0]);        cout << maxElement(A, n);        return 0; }

## Java

 // Java program to find a number that  // divides maximum array elements  import java.util.*; class Solution { static final int MAXN=100001;       // stores smallest prime factor for every number  static int spf[]= new int[MAXN];       // Calculating SPF (Smallest Prime Factor) for every  // number till MAXN.  // Time Complexity : O(nloglogn)  static void sieve()  {      spf[1] = 1;      for (int i = 2; i < MAXN; i++)               // marking smallest prime factor for every          // number to be itself.          spf[i] = i;           // separately marking spf for every even      // number as 2      for (int i = 4; i < MAXN; i += 2)          spf[i] = 2;           for (int i = 3; i * i < MAXN; i++) {          // checking if i is prime          if (spf[i] == i) {              // marking SPF for all numbers divisible by i              for (int j = i * i; j < MAXN; j += i)                       // marking spf[j] if it is not                  // previously marked                  if (spf[j] == j)                      spf[j] = i;          }      }  }       // A O(log n) function returning primefactorization  // by dividing by smallest prime factor at every step  static Vector getFactorization(int x)  {      Vector ret= new Vector();      while (x != 1) {          int temp = spf[x];          ret.add(temp);          while (x % temp == 0)              x = x / temp;      }      return ret;  }       // Function to find a number that  // divides maximum array elements  static int maxElement(int A[], int n)  {      // precalculating Smallest Prime Factor      sieve();           // Hash to store frequency of each divisors      Map m= new HashMap();           // Traverse the array and get spf of each element      for (int j = 0; j < n; ++j) {               // calling getFactorization function          Vector p = getFactorization(A[j]);               for (int i = 0; i < p.size(); i++)              m.put(p.get(i),m.get(p.get(i))==null?0:m.get(p.get(i))+1);      }           int cnt = 0, ans = 10000000;      // Returns Set view              Set< Map.Entry< Integer,Integer> > st = m.entrySet();                 for (Map.Entry< Integer,Integer> me:st)         {          if (me.getValue() >= cnt) {              cnt = me.getValue();              if(ans > me.getKey())              ans = me.getKey() ;             else             ans = ans;          }      }           return ans;  }       // Driver program  public static void main(String args[]) {      int A[] = { 2, 5, 10 };      int n =A.length;           System.out.print(maxElement(A, n));          }  } //contributed by Arnab Kundu

## Python3

# Python3 program to find a number that
# divides maximum array elements
import math as mt

MAXN = 100001

# stores smallest prime factor for
# every number
spf = [0 for i in range(MAXN)]

# Calculating SPF (Smallest Prime Factor)
# for every number till MAXN.
# Time Complexity : O(nloglogn)
def sieve():

spf[1] = 1
for i in range(2, MAXN):

# marking smallest prime factor for
# every number to be itself.
spf[i] = i

# separately marking spf for every
# even number as 2
for i in range(4, MAXN, 2):
spf[i] = 2

for i in range(3, mt.ceil(mt.sqrt(MAXN + 1))):

# checking if i is prime
if (spf[i] == i):

# marking SPF for all numbers divisible by i
for j in range(2 * i, MAXN, i):

# marking spf[j] if it is not
# previously marked
if (spf[j] == j):
spf[j] = i

# A O(log n) function returning primefactorization
# by dividing by smallest prime factor at every step
def getFactorization (x):

ret = list()
while (x != 1):
temp = spf[x]
ret.append(temp)
while (x % temp == 0):
x = x //temp

return ret

# Function to find a number that
# divides maximum array elements
def maxElement (A, n):

# precalculating Smallest Prime Factor
sieve()

# Hash to store frequency of each divisors
m = dict()

# Traverse the array and get spf of each element
for i in range(n):

# calling getFactorization function
p = getFactorization(A[i])

for i in range(len(p)):
if p[i] in m.keys():
m[p[i]] += 1
else:
m[p[i]] = 1

cnt = 0
ans = 10**9+7

for i in m:
if (m[i] >= cnt):
cnt = m[i]
if ans > i:
ans = i
else:
ans = ans

return ans

# Driver Code
A = [2, 5, 10 ]
n = len(A)

print(maxElement(A, n))

# This code is contributed by Mohit kumar 29

Output:

2

Time Complexity: O(N*log(N))

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