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