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  1. function longest_palindrome(s) {
  2.     const palindromes = new Set();
  3.     const finishedPalindromes = [];
  4.  
  5.     const getLength = (i, c, isEven) => (i - c) * 2 + (isEven ? 0 : 1);
  6.     const getPaired = (i, c, isEven) => i - (i - c) * 2 + (isEven ? 1 : 0);
  7.  
  8.     for (let i = 0; i < s.length + 1; i++) {
  9.       for (const p of palindromes) {
  10.         const {isEven, center} = p;
  11.         const probLength = getLength(i, center, isEven);
  12.         if (i >= s.length || s[getPaired(i, center, isEven)] !== s[i]) {
  13.           const length = probLength - 2;
  14.           palindromes.delete(p);
  15.           finishedPalindromes.push({...p, length, text: s.slice(getPaired(i - 1, center, isEven), i) })
  16.         }
  17.       }
  18.       if (i >= s.length) {
  19.         continue;
  20.       }
  21.       if (s[i + 1] === s[i - 1]) {
  22.         palindromes.add({isEven: false, center: i});
  23.       }
  24.       if (s[i] === s[i - 1]) {
  25.         palindromes.add({isEven: true, center: i - 1});
  26.       }
  27.     }
  28.  
  29.     let longest = s.length === 0 ?
  30.         {center: 0, length: 0, isEven: true, text: ''} :
  31.         {center: 0, length: 1, isEven: false, text: s[0]};
  32.     for (const p of finishedPalindromes) {
  33.       if (p.length > longest.length) {
  34.         longest = p;
  35.       }
  36.     }
  37.     return longest.text;
  38.   }
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