Leetcode: a bitmask dp problem

Construct Smallest Number From DI String

You are given a 0-indexed string pattern of length n consisting of the characters 'I' meaning increasing and 'D' meaning decreasing.

A 0-indexed string num of length n + 1 is created using the following conditions:

  • num consists of the digits '1' to '9', where each digit is used at most once.
  • If pattern[i] == 'I', then num[i] < num[i + 1].
  • If pattern[i] == 'D', then num[i] > num[i + 1].

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