Add Largest Prime Palindrome to (L-A).

Author: tsongtheng

L-A/0015 Largest Prime Palindrome/LargestPrimePalindrome.js

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+function isPrime(num) {
+  if (num <= 1) {
+    return false;
+  }
+  if (num <= 3) {
+    return true;
+  }
+  if (num % 2 === 0 || num % 3 === 0) {
+    return false;
+  }
+  let i = 5;
+  while (i * i <= num) {
+    if (num % i === 0 || num % (i + 2) === 0) {
+      return false;
+    }
+    i += 6;
+  }
+  return true;
+}
+
+function isPalindrome(num) {
+  const numStr = num.toString();
+  const reversedStr = numStr.split("").reverse().join("");
+  return numStr === reversedStr;
+}
+
+function largestPrimePalindrome(n) {
+  for (let i = n; i >= 2; i--) {
+    if (isPrime(i) && isPalindrome(i)) {
+      return i;
+    }
+  }
+  return null; // No prime palindrome found in the range
+}
+
+// Test cases
+console.log(largestPrimePalindrome(100)); // 7
+console.log(largestPrimePalindrome(50)); // 7
+console.log(largestPrimePalindrome(10)); // 7

L-A/0015 Largest Prime Palindrome/README.md

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+# 0015 Largest Prime Palindrome ( L-A )
+
+## Problem
+
+Write a JavaScript function called largestPrimePalindrome that takes an integer **n** as input and returns the largest prime palindrome that is less than or equal to **n**. A prime palindrome is a number that is both prime and reads the same forward and backward.
+
+**Example**
+
+```
+If the input n is 100, the function should return 7 because the largest prime palindrome less than or equal to 100 is 7.
+
+If the input n is 50, the function should return 7 because 7 is the largest prime palindrome less than or equal to 50.
+
+If the input n is 10, the function should return 7 because 7 is the largest prime palindrome less than or equal to 10.
+```
+
+## Solutions
+
+```javascript
+function isPrime(num) {
+  if (num <= 1) {
+    return false;
+  }
+  if (num <= 3) {
+    return true;
+  }
+  if (num % 2 === 0 || num % 3 === 0) {
+    return false;
+  }
+  let i = 5;
+  while (i * i <= num) {
+    if (num % i === 0 || num % (i + 2) === 0) {
+      return false;
+    }
+    i += 6;
+  }
+  return true;
+}
+
+function isPalindrome(num) {
+  const numStr = num.toString();
+  const reversedStr = numStr.split("").reverse().join("");
+  return numStr === reversedStr;
+}
+
+function largestPrimePalindrome(n) {
+  for (let i = n; i >= 2; i--) {
+    if (isPrime(i) && isPalindrome(i)) {
+      return i;
+    }
+  }
+  return null; // No prime palindrome found in the range
+}
+
+// Test cases
+console.log(largestPrimePalindrome(100)); // 7
+console.log(largestPrimePalindrome(50)); // 7
+console.log(largestPrimePalindrome(10)); // 7
+```
+
+## How it works
+
+1. The isPrime function checks whether a given number is prime using a primality test that efficiently eliminates multiples of 2 and 3 and then checks for divisibility by numbers of the form 6k ± 1.
+
+2. The isPalindrome function checks whether a given number is a palindrome by converting it to a string, reversing the string, and comparing it to the original string.
+
+3. The largestPrimePalindrome function starts from the given number n and counts down to 2, checking each number in reverse order. It uses the isPrime and isPalindrome functions to find the largest prime palindrome in the given range.
+
+## References
+
+- [ChatGPT](https://chat.openai.com/)
+
+## Problem Added By
+
+- [Tipchan](https://github.com/tsongtheng)
+
+## Contributing
+
+Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change.
+
+Please make sure to update tests as appropriate.