Array and Strings

For an array-and-string question, first establish whether the object is a mutable character array or an immutable string in the chosen language. Ask whether characters are lowercase English letters, arbitrary bytes, or Unicode code points, because this changes safe indexing and memory use. Many interview tasks reduce to a linear scan with two pointers: maintain a window and counts, expand the right edge, then shrink the left edge only while the invariant is violated. For an anagram check, compare frequency counts; for a substring requirement, track the number of satisfied categories rather than rescanning the whole map each move. Test empty input, repeated characters, a result at either endpoint, and characters outside ASCII if applicable. Avoid repeated string concatenation in a loop because it can make a nominal O(n) algorithm O(n squared). A typical window solution is O(n) time and O(k) space, where k is the tracked alphabet size.