A14578 | Two Sorts

A string $a$ is lexicographically smaller than a string $b$ if and only if one of the following holds:

- $a$ is a prefix of $b$ , but $a \ne b$ ;
- in the first position where $a$ and $b$ differ, the string $a$ has a letter that appears earlier in the alphabet than the corresponding letter in $b$ .

For example, $42$ is lexicographically smaller than $6$ , because they differ in the first digit, and $4 < 6$ ; $42 < 420$ , because $42$ is a prefix of $420$ .

Let's denote $998244353$ as $M$ .

In the first example, array $a$ is equal to $[1, 2, 3]$ .

- $(1 - 1) \mod M = 0 \mod M = 0$
- $(2 - 2) \mod M = 0 \mod M = 0$
- $(3 - 3) \mod M = 0 \mod M = 0$

As a result, $(0 + 0 + 0) \mod 10^9 + 7 = 0$

In the second example, array $a$ is equal to $[1, 10, 11, 12, 2, 3, 4, 5, 6, 7, 8, 9]$ .

- $(1 - 1) \mod M = 0 \mod M = 0$
- $(2 - 10) \mod M = (-8) \mod M = 998244345$
- $(3 - 11) \mod M = (-8) \mod M = 998244345$
- $(4 - 12) \mod M = (-8) \mod M = 998244345$
- $(5 - 2) \mod M = 3 \mod M = 3$
- $(6 - 3) \mod M = 3 \mod M = 3$
- $(7 - 4) \mod M = 3 \mod M = 3$
- $(8 - 5) \mod M = 3 \mod M = 3$
- $(9 - 6) \mod M = 3 \mod M = 3$
- $(10 - 7) \mod M = 3 \mod M = 3$
- $(11 - 8) \mod M = 3 \mod M = 3$
- $(12 - 9) \mod M = 3 \mod M = 3$

As a result, $(0 + 998244345 + 998244345 + 998244345 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3) \mod 10^9 + 7$ $=$ $2994733059 \mod 10^9 + 7$ $=$ $994733045$