Please solve the programming task below using a self-contained code snippet in a markdown code block.

You are given a string s of length n where s[i] is either:

'D' means decreasing, or
'I' means increasing.

A permutation perm of n + 1 integers of all the integers in the range [0, n] is called a valid permutation if for all valid i:

If s[i] == 'D', then perm[i] > perm[i + 1], and
If s[i] == 'I', then perm[i] < perm[i + 1].

Return the number of valid permutations perm. Since the answer may be large, return it modulo 109 + 7.
 
Please complete the following python code precisely:
```python
class Solution:
    def numPermsDISequence(self, s: str) -> int:
```