C++ Program to Count rotations divisible by 4
Last Updated :
17 Aug, 2023
Given a large positive number as string, count all rotations of the given number which are divisible by 4.
Examples:
Input: 8
Output: 1
Input: 20
Output: 1
Rotation: 20 is divisible by 4
02 is not divisible by 4
Input : 13502
Output : 0
No rotation is divisible by 4
Input : 43292816
Output : 5
5 rotations are : 43292816, 16432928, 81643292
92816432, 32928164
For large numbers it is difficult to rotate and divide each number by 4. Therefore, ‘divisibility by 4’ property is used which says that a number is divisible by 4 if the last 2 digits of the number is divisible by 4. Here we do not actually rotate the number and check last 2 digits for divisibility, instead we count consecutive pairs (in circular way) which are divisible by 4.
Illustration:
Consider a number 928160
Its rotations are 928160, 092816, 609281, 160928,
816092, 281609.
Now form pairs from the original number 928160
as mentioned in the approach.
Pairs: (9,2), (2,8), (8,1), (1,6),
(6,0), (0,9)
We can observe that the 2-digit number formed by the these
pairs, i.e., 92, 28, 81, 16, 60, 09, are present in the last
2 digits of some rotation.
Thus, checking divisibility of these pairs gives the required
number of rotations.
Note: A single digit number can directly
be checked for divisibility.
Below is the implementation of the approach.
C++
#include <bits/stdc++.h>
using namespace std;
int countRotations(string n)
{
int len = n.length();
if (len == 1)
{
int oneDigit = n.at(0)- '0' ;
if (oneDigit%4 == 0)
return 1;
return 0;
}
int twoDigit, count = 0;
for ( int i=0; i<(len-1); i++)
{
twoDigit = (n.at(i)- '0' )*10 + (n.at(i+1)- '0' );
if (twoDigit%4 == 0)
count++;
}
twoDigit = (n.at(len-1)- '0' )*10 + (n.at(0)- '0' );
if (twoDigit%4 == 0)
count++;
return count;
}
int main()
{
string n = "4834" ;
cout << "Rotations: " << countRotations(n) << endl;
return 0;
}
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Output:
Rotations: 2
Time Complexity : O(n) where n is number of digits in input number.
Auxiliary Space: O(1)
Please refer complete article on Count rotations divisible by 4 for more details!
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