Longest Palindromic Substring

String ★★★☆

Find the longest contiguous substring of a 74-character lowercase string that reads the same forwards and backwards.

A1 contains a 74-character lowercase string — a transmission with one embedded palindromic callsign hidden in noise. Return the longest contiguous substring that reads the same forwards and backwards.

The string has C(74, 2) = 2,701 substrings of length ≥ 2. Eyeballing that for symmetry doesn’t work — the formula has to enumerate, test, and pick the max. For this input the longest palindrome is unique.

Input

Range A1

	A
1	xojbnpxmuilgelneugukbmfqqiwgusumamenodonemamusfbtkwfcujjgvzlhtkzvxoevnwenw

Your output

Hint

Generate every substring with MAKEARRAY (rows = start, cols = end), flatten with TOCOL skipping blanks, BYROW-test each one with a palindrome checker, then FILTER to the longest.

Solution

=LET(
  str, A1,
  n, LEN(str),
  rev, LAMBDA(txt, LET(ln,LEN(txt), CONCAT(MID(txt,SEQUENCE(ln,,ln,-1),1)) = txt)),
  m, TOCOL(MAKEARRAY(n, n, LAMBDA(r,c, IF(AND(c>r,c<>r), MID(str,r,c-r+1), ""))), 3),
  b, BYROW(m, LAMBDA(br, IFERROR(rev(INDEX(br,1,1)), FALSE))),
  palindromes, FILTER(m, b*LEN(m)),
  FILTER(palindromes, LEN(palindromes) = MAX(LEN(palindromes)))
)