check subsequence of length L is palindrome or not
for ( int L = 2 ; L <= N ; L ++ ) { for ( int i = 0 ; i <= N - L ; i ++ ) { int k = L + i - 1 ; if ( str [ i ] == str [ k ] ) cps [ i ] [ k ] = cps [ i ] [ k - 1 ] + cps [ i + 1 ] [ k ] + 1 ; else cps [ i ] [ k ] = cps [ i ] [ k - 1 ] + cps [ i + 1 ] [ k ] - cps [ i + 1 ] [ k - 1 ] ; } }