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_866.java
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_866.java
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import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
/**
* LeetCode 867 - Prime Palindrome
* <p>
* Fast Primality Test
* <p>
* - Use numerical calculation to revert a number when concatenate each palindrome.
* - Use fast primality test as the number range is pretty small.
*/
public class _866 {
static List<Integer> cand = new ArrayList<>();
static int CERTAINTY = 32;
static {
cand.addAll(Arrays.asList(2, 3, 5, 7));
// Odd-length
loop:
{
for (int prefix = 1; ; prefix++) {
for (int mid = 0; mid < 10; mid++) {
int len = (int) Math.log10(prefix) + 1;
long palindrome = prefix * 10 + mid, tmp = prefix;
for (int i = 0; i < len; i++) {
palindrome = palindrome * 10 + tmp % 10;
tmp /= 10;
}
if (palindrome > Integer.MAX_VALUE) {
break loop;
}
if (BigInteger.valueOf(palindrome).isProbablePrime(CERTAINTY)) {
cand.add((int) palindrome);
}
}
}
}
// Even-length
for (int prefix = 1; ; prefix++) {
int len = (int) Math.log10(prefix) + 1;
long palindrome = prefix, tmp = prefix;
for (int i = 0; i < len; i++) {
palindrome = palindrome * 10 + tmp % 10;
tmp /= 10;
}
if (palindrome > Integer.MAX_VALUE) {
break;
}
if (BigInteger.valueOf(palindrome).isProbablePrime(CERTAINTY)) {
cand.add((int) palindrome);
}
}
Collections.sort(cand);
}
public int primePalindrome(int N) {
for (int i : cand) {
if (i >= N) {
return i;
}
}
throw new RuntimeException();
}
}