409. Longest Palindrome

Given a string s of upper- and lowercase letters, return the length of the longest palindrome that can be built from its letters (case-sensitive).

Walk the string, keeping the set of characters seen an odd number of times. Seeing one already in the set completes a pair (+2).

1function longestPalindrome(s: string): number {
2  const open = new Set<string>();
3  let len = 0;
4  for (const c of s) {
5    if (open.has(c)) { open.delete(c); len += 2; }
6    else open.add(c);
7  }
8  return len + (open.size ? 1 : 0);
9}

len = 0

s

unpaired

empty

No unpaired characters yet.

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