String Manipulation

String manipulation should begin with the exact transformation: deletion, replacement, compression, reversal, tokenization, or normalization. Because strings are immutable in many runtimes, repeated concatenation can make an otherwise linear transformation quadratic. Use a builder, character array, or list of fragments, and keep explicit read and write positions if transforming in place. For expansion tasks, work backward when the destination has enough capacity; for filtering, write accepted characters forward. Be precise about whether indices refer to bytes, code units, code points, or user-visible graphemes. Test empty input, one character, consecutive separators, and replacement text that grows or shrinks the result. A single pass is generally O(n) time, with O(n) output space unless true in-place mutation is supported. Do not silently trim, lowercase, or normalize unless the contract says so.