text: If current prefix satisfies the given conditions
code: if ( k . length == 1 || ( k [ 0 ] == k [ k . length - 2 ] && k [ k . length - 1 ] - k [ k . length - 2 ] == 1 ) || ( k [ 0 ] == 1 && k [ 1 ] == k [ k . length - 1 ] ) ) ans = index ; }