903. Valid Permutations for DI Sequence LeetCode Solution

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Valid Permutations for DI Sequence 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.

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'.

Valid Permutations for DI Sequence Solutions

Time: O(n)
Space: O(n)

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