PROJECT EULER P4 cannot get the answer

debugcn Published at Dev

David Jiang

Problem 4

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99. Find the largest palindrome made from the product of two 3-digit numbers.

here is my code below. I try my code on the product of two 2-digit numbers, it prints 9009 (correct answer). but for the 3 digit numbers, the correct answer is 906609, my answer is 956459. I don't know what 's wrong with my code, please help. Thanks.

let num;//product of two numbers
let paNum=[];//array for palindrome number
for(i=100;i<1000;i++)
{
    for (j=100;j<1000;j++)
    {
        num = i*j;
        str= num.toString();//change the number to a string
        if (str.substr(0,1) == str.substr(-1,1) && str.substr(1,1) == str.substr(-2,1))
        {
            paNum.push(num);
        }
        else{};
    };

};
console.log(paNum);
console.log(Math.max(...paNum));

menaka_

I got the correct answer with the following modification.

for(i=100;i<1000;i++) {
    for (j=100;j<1000;j++){
        num = i*j;
        str= num.toString();//change the number to a string
        let len = str.length - 1;
        let checkedDigit = [];

        //Iterate the string from 0 to string length/2 and check whether it is a palindrom.
        for (k = 0; k < str.length/2; k++) {
            if (str.substr(k, 1) === str.substr(len - (len + 1 + k), 1)) {
                //If the two corresponding digits are equal, add a true. (Else false)
                checkedDigit[k] = true;
            } else {
                checkedDigit[k] = false;
            }
        }

  function checkDigit(val) {
    return val === true;
  }

  //Check whether all the elements in the checkedDigit array are true.
  if (checkedDigit.every(checkDigit)) {
    paNum.push(num);
  }

};
};
console.log(paNum);
console.log(Math.max(...paNum));

Collected from the Internet

Please contact [email protected] to delete if infringement.