Longest Palindromic Substring / visualized

Every other two-pointer machine on this list starts at the two ends and closes inward. This one inverts it: the pointers start together and move apart, because a palindrome is defined outward from its middle. The trap is counting the middles. A string of length n has 2n−1 centers, not n — n of them sit on a character (odd-length palindromes) and n−1 sit in the gap between two characters (even-length ones, which have no middle character at all). Miss the gaps and "cbbd" silently answers "b". From each center, push l left and r right while s[l] == s[r]; the first mismatch ends that center, and the longest span ever confirmed is the answer.