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Find next palindrome prime in C++
In this problem, we are given an element N. We need to find the next palindrome prime.
Problem Description − We need to find the smallest prime number which is also a palindrome number, greater than N.
Palindrome Number is a number in which the numbers are the same in both directions.
Prime Number is a number if its only factors are 1 and itself.
Let’s take an example to understand the problem,
Input
N = 12
Output
101
Explanation
The series of palindromes greater than 12 are 22, 33, 44, 55, 66, 77, 88, 99, 101… out of these the smallest palindrome is 101.
Solution Approach
A simple solution to the problem is by finding all palindromes greater than N that are primes.
A more efficient solution is by finding the even digit palindrome which is multiple of 11.
Here is a proof of this solution,
11% 11 = 0 1111% 11 = 0
Using this we will find palindrome with even digits −
xyzzyx % 11 = 0, which makes all even number digits not palindrome.
Program to illustrate the working of our solution,
Example
#include <iostream>
#include <string>
using namespace std;
bool isPrime(int num) {
if (num < 2 || num % 2 == 0)
return num == 2;
for (int i = 3; i * i <= num; i += 2)
if (num % i == 0)
return false;
return true;
}
int primePalindrome(int N) {
if (8 <= N && N <= 11)
return 11;
for (int x = 1; x < 100000; ++x) {
string s = to_string(x), r(s.rbegin(), s.rend());
int y = stoi(s + r.substr(1));
if (y >= N && isPrime(y))
return y;
}
return -1;
}
int main() {
int N = 432;
cout<<"The next prime palindrome is "<<findNextPrimePallindrome(432);
return 0;
}Output
The next number with same set of digits is 92543
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