If current prefix satisfies the given conditions
if ( k . size ( ) == 1 || ( k . get ( 0 ) == k . get ( k . size ( ) - 2 ) && k . get ( k . size ( ) - 1 ) - k . get ( k . size ( ) - 2 ) == 1 ) || ( k . get ( 0 ) == 1 && k . get ( 1 ) == k . get ( k . size ( ) - 1 ) ) ) ans = index ; }