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Question

I got this to work after tinkering with it, but am unclear on some of the -1s and +1s going on. my code below as well as my questions in the comments. Also recapping the questions up here:

1. In the return statement inside the "centeredPalindrome" helper function why is it "left + 1"? Is that because you are expanding but only care about what's "in the expansion", not the outer limit?
2. In that same return statement, why right and not right +1? is it because you are doing "length" and NOT "length-1" in the while condition?
3. if it is odd, we expand one extra to the left - why? is it because an odd palindrome will always have "one extra at the beginning"?

---

var longestPalindrome = function(string){
  var length = string.length;
  var result = "";

  //helper function
  var centeredPalindrome = function(left, right){
    //while loop with conditions for it being a palindrome. iterate to left/right while those conditions are met:
    while(left>= 0 && right < length&& string[left] === string[right]){
      left--;
      right++;
    }

    //why right and not right + 1? is it because you are doing length (and NOT length -1) in the while loop?
    //why left + 1? Is that because you are expanding but only care about what's "in the expansion", not the outer limit?
    return string.slice(left + 1, right);
  }

  //iterate through the string and apply the helper function

  for (var i = 0; i < string.length; i++) {
    //handle case for it being odd or even
    var evenPal = centeredPalindrome(i, i);
    // if it is odd, we expand one extra to the left via "i -1" - why? is it because an odd palindrome will always have "one extra at the beggining"?
    var oddPal = centeredPalindrome(i-1, i);


    //overwrite the result with the longest one between them
    if(oddPal.length > result.length){
      result = oddPal;
    }

    if(evenPal.length > result.length){
      result = evenPal;
    }
  };

  //return the final result
  return result;
}


console.log(longestPalindrome("racecar"));

// returns "racecar" if I change the return inside "centerPalindrome" to string.slice(left, right), this becomes:
//"ra"
console.log(longestPalindrome("radar")); // returns "radar"
console.log(longestPalindrome("abba")); // returns "abba"

---

probably better to name the variables like this per @DrewGaynor:

    var oddPal = centeredPalindrome(i - 1, i + 1);
    var evenPal = centeredPalindrome(i, i + 1);

in the case of the odd palindrome, you want to look to the left and to the right of the center, as below.

var oddPal = centeredPalindrome(i - 1, i + 1);
        racecar
           ^
           |
           Center is one character because the string has an odd length (7 characters)

in the case of the even palindrome, you want to look at the center which is two chars long, to do this, you need to account for the extra length of the center. you could have also done i-1 for the "left" instead of i+1 for the "right". but you don't want to do it for both since then you will be looking at a three letter center or starting the left at -1!

     var evenPal = centeredPalindrome(i, i + 1);


         abba
          ^^
          |
          Center is two characters because the string has an even length (4 characters)

Answer 1

In the return statement inside the "centeredPalindrome" helper function why is it "left + 1"? Is that because you are expanding but only care about what's "in the expansion", not the outer limit?

In that same return statement, why right and not right +1? is it because you are doing "length" and NOT "length-1" in the while condition?

The left and right variables are the left and right outer bounds of the substring. The desired output is everything between those bounds. To illustrate:

abcd_racecar-efgh
    ^       ^
    |       |
    |       Final value of "right" (12)
    Final value of "left" (4)

The arguments of the slice function are the start index (inclusive, so the actual index of the start of the desired output) and end index (exclusive, so the index immediately following the end of the desired output). The right value is already where we want it, but the left value needs to be incremented by 1 to be correct.

"abcd_racecar-efgh".slice(5, 12); //Output is "racecar"

If it is odd, we expand one extra to the left - why? is it because an odd palindrome will always have "one extra at the beginning"?

This is done because the center of the palindrome could be two characters if it has an even length, or one character if it has an odd length (which actually seems to me to contradict the variable names). To illustrate:

racecar
   ^
   |
   Center is one character because the string has an odd length (7 characters)

 abba
  ^^
  |
  Center is two characters because the string has an even length (4 characters)