5. Longest Palindromic Substring

Return the longest palindromic substring of s.

Every palindrome mirrors around a center: a character (odd length) or a gap between two characters (even length). Expand outward from each of the 2n − 1 centers.

1function longestPalindrome(s: string): string {
2  let lo = 0, hi = 0;
3  const expand = (l: number, r: number) => {
4    while (l >= 0 && r < s.length && s[l] === s[r]) { l--; r++; }
5    if (r - l - 1 > hi - lo + 1) { lo = l + 1; hi = r - 1; }
6  };
7  for (let i = 0; i < s.length; i++) {
8    expand(i, i);
9    expand(i, i + 1);
10  }
11  return s.slice(lo, hi + 1);
12}

best = "b"

s

l,r

best = first character.

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