Leetcode 2375. Construct Smallest Number From DI String

Given a pattern of 'I' (increasing) and 'D' (decreasing) of length n, produce the lexicographically smallest string of distinct digits 1..9 of length n+1 whose adjacent comparisons match the pattern. Core challenge is to greedily assign the smallest unused digits while handling runs of consecutive 'D's (which force local reversals/stacking) so the overall sequence remains minimal.