16371번 - AB-Strings 서브태스크스페셜 저지

There are two strings s and t, consisting only of letters a and b. You can make the following operation several times: choose a prefix of s, a prefix of t and swap them. Prefixes can be empty, also a prefix can coincide with a whole string.

Your task is to find a sequence of operations after which one of the strings consists only of a letters and the other consists only of b letters. The number of operations should be minimized, but solutions that find non-optimal sequences will still get some points. Read the scoring section for more detailed information.

The first line contains a string s (1 ≤ |s| ≤ 2 · 10⁵).

The second line contains a string t (1 ≤ |t| ≤ 2 · 10⁵).

Here |s| and |t| denote the lengths of s and t, respectively. It is guaranteed that at least one of the strings contains at least one a letter and at least one of the strings contains at least one b letter.

The first line should contain a single integer n (0 ≤ n ≤ 5 · 10⁵)—the number of operations.

Each of the next n lines should contain two space-separated integers a_i, b_i—the lengths of prefixes of s and t to swap, respectively.

If there are multiple possible solutions, you can print any of them.

• 1 ≤ |s|, |t| ≤ 6
• s and t combined contain exactly one letter a

• 1 ≤ |s|, |t| ≤ 6

• 1 ≤ |s|, |t| ≤ 50

• 1 ≤ |s|, |t| ≤ 250

• 1 ≤ |s|, |t| ≤ 2000

• 1 ≤ |s|, |t| ≤ 2 · 10⁵

In the first example, you can solve the problem in two operations:

1. Swap the prefix of the first string with length 1 and the prefix of the second string with length 0. After this swap, you’ll have strings ab and bbb.
2. Swap the prefix of the first string with length 1 and the prefix of the second string with length 3. After this swap, you’ll have strings bbbb and a.

In the second example, the strings are already appropriate, so no operations are needed.