freeCodeCamp/guide/english/certifications/coding-interview-prep/project-euler/problem-4-largest-palindrome-product/index.md

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Problem 4: Largest palindrome product

Problem 4: Largest palindrome product

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Problem Explanation

• A palindromic number is the one that when reversed reads the same.
• The largest number obtained from product of two 3 digit number is 999 * 999, so we can make a loop that starts by producting the largest number and check if that number is palindromic or not.

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Solutions

Solution 1 (Click to Show/Hide)

function largestPalindromeProduct(n) {
  //To get the maximum n digit number, + operator type castes String to Number type
  let max = +[...Array(n)].reduce((a, c) => (a += 9), "");

  //Next we get minimum n digit number from the max
  let min = (max + 1) / 10;

  //To store the result
  let res = [];

  //Starting the loop from max to min
  for (let i = max; i >= min; i--) {
    //Another loop
    for (let j = max; j >= min; j--) {
      //Getting the product
      let num = i * j;

      //Reversing the number
      let numReverse = [...String(num)].reverse().join("");

      //Checking for palindromic number
      if (num == numReverse) {
        //Pushing the number into array and breaking the loop for efficiency
        res.push(num);
        break;
      }
    }
  }

  // Returning the maximum of the result array
  return Math.max(...res);
}

Relevant Links

• Wikipedia