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Leetcode 2645. Minimum Additions to Make Valid String
Given a string over {a,b,c}, compute the minimum number of insertions needed to turn it into a concatenation of "abc" blocks by aligning its characters to the cyclic pattern "a"→"b"→"c". The core challenge is pattern matching against this fixed cycle — solvable by a simple greedy scan that counts missing letters for each expected position.
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