Valid Permutations for DI Sequence
You are given a string s of length n where s[i] is either: A permutation perm of n + 1 integers of all the integers in the range [0, n] is called a valid permutation if for all...
You are given a string s of length n where s[i] is either:
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:
Return the number of valid permutations perm. Since the answer may be large, return it modulo 10^9 + 7.
Example 1
Input: s = "DID"
Output: 5
Explanation: The 5 valid permutations of (0, 1, 2, 3) are: (1, 0, 3, 2) (2, 0, 3, 1) (2, 1, 3, 0) (3, 0, 2, 1) (3, 1, 2, 0)
Example 2
Input: s = "D"
Output: 1
Constraints
- n == s.length
- 1 <= n <= 200
- s[i] is either 'I' or 'D'.
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