B'This is the hard version of the problem. The difference between the versions is the constraint on n and the required number of operations. You can make hacks only if all versions of the problem are solved. There are two binary strings a and b of length n (a binary string is a string consisting of symbols 0 and 1 ). In an operation, you select a prefix of a , and simultaneously invert the bits in the prefix ( 0 changes to 1 and 1 changes to 0 ) and reverse the order of the bits in the prefix. For example, if a=001011 and you select the prefix of length 3 , it becomes 011011 . Then if you select the entire string, it becomes 001001 . Your task is to transform the string a into b in at most 2n operations. It can be proved that it is always possible. The first line contains a single integer t ( 1 <= t <= 1000 ) -- the number of test cases. Next 3t lines contain descriptions of test cases. The first line of each test case contains a single integer n ( 1 <= n <= 10^5 ) -- the length of the binary strings. The next two lines contain two binary strings a and b of length n . It is guaranteed that the sum of n across all test cases does not exceed 10^5 . For each test case, output an integer k ( 0 <= k <= 2n ), followed by k integers p_1, ldots,p_k ( 1 <= p_i <= n ). Here k is the number of operations you use and p_i is the length of the prefix you flip in the i -th operation. In the first test case, we have 01 to 11 to 00 to 10 . In the second test case, we have 01011 to 00101 to 11101 to 01000 to 10100 to 00100 to 11100 . In the third test case, the strings are already the same. Another solution is to flip the prefix of length 2 , which will leave a unchanged. '...