Find the Occurrence of First Almost Equal Substring

For the usual formulation, find the earliest length-m substring of s that differs from pattern in at most one position, so only substitutions are allowed and candidate windows have equal length. The simple approach scans every start and stops counting mismatches once it exceeds one; it is O((n-m+1)*m), which may be adequate only for small constraints. For a linear-style approach, compute longest common prefix matches between pattern and every alignment with a Z-function or rolling hashes, then for an alignment skip the matching prefix, skip one mismatched character, and use reversed-string suffix matches to verify the remaining suffix. State collision handling if hashes are used. Clarify that insertion or deletion changes the problem into edit distance and needs different DP. Test exact matches, one mismatch at either edge, two mismatches, pattern longer than text, and repeated prefixes. Prefix/suffix preprocessing gives O(n+m) time and O(n+m) space.