Maximum length of a sub-array with ugly numbers in C++

Problem statement

Given an array arr[] of N elements (0 ≤ arr[i] ≤ 1000). The task is to find the maximum length of the sub-array that contains only ugly numbers.

Ugly numbers are numbers whose only prime factors are 2, 3 or 5.

For Example below are the few number from series: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15,…

Example

If input array is {1, 2, 7, 9, 120, 810, 374} then answer is 3 as −

Longest possible sub-array of ugly number sis {9, 120, 810}

Algorithm

• Take a unordered_set, and insert all the ugly numbers which are less than 1000 in the set
• Traverse the array with two variables named current_max and max_so_far.
• Check for each element if it is present in the set
• If an ugly number is found, then increment current_max and compare it with max_so_far
• If current_max > max_so_far then max_so_far = current_max.
• Every time a non-ugly element is found, reset current_max = 0.

Example

 Live Demo

#include <bits/stdc++.h>
using namespace std;
unsigned getUglyNumbers(int n) {
   int ugly[n];
   int i2 = 0, i3 = 0, i5 = 0;
   int next_multiple_of_2 = 2;
   int next_multiple_of_3 = 3;
   int next_multiple_of_5 = 5;
   int next_ugly_no = 1;
   ugly[0] = 1;
   for (int i = 1; i < n; i++) {
      next_ugly_no = min(next_multiple_of_2, min(next_multiple_of_3, next_multiple_of_5));
      ugly[i] = next_ugly_no;
      if (next_ugly_no == next_multiple_of_2) {
         i2 = i2 + 1;
         next_multiple_of_2 = ugly[i2] * 2;
      }
      if (next_ugly_no == next_multiple_of_3) {
         i3 = i3 + 1;
         next_multiple_of_3 = ugly[i3] * 3;
      }
      if (next_ugly_no == next_multiple_of_5) {
         i5 = i5 + 1;
         next_multiple_of_5 = ugly[i5] * 5;
      }
   }
   return next_ugly_no;
}
int maxUglySubarray(int arr[], int n) {
   unordered_set<int> s;
   int i = 1;
   while (1) {
      int next_ugly_number = getUglyNumbers(i);
      if (next_ugly_number > 1000)
         break;
      s.insert(next_ugly_number);
      i++;
   }
   int current_max = 0, max_so_far = 0;
   for (int i = 0; i < n; i++) {
      if (s.find(arr[i]) == s.end())
         current_max = 0;
      else {
         current_max++;
         max_so_far = max(current_max,
         max_so_far);
      }
   }
   return max_so_far;
}
int main() {
   int arr[] = {1, 2, 7, 9, 120, 810, 374};
   int n = sizeof(arr) / sizeof(arr[0]);
   cout << "Maximum sub-array size of consecutive ugly numbers = " << maxUglySubarray(arr, n) << endl;
   return 0;
}

Output

When you compile and execute above program. It generates following output −

Maximum sub-array size of consecutive ugly numbers = 3