Strings

Strings invite several recurring patterns. Use two pointers for reversal and palindrome checks, a frequency map for anagrams and uniqueness, a sliding window for substring constraints, a stack for balanced syntax, and prefix-function or rolling-hash techniques for repeated pattern search. Before selecting one, clarify whether the input is ASCII, Unicode code points, or user-visible graphemes, and whether case, punctuation, and whitespace count. In many languages strings are immutable, so construct changed output with a builder or fragments rather than appending character by character to a new immutable value. A linear scan is O(n) time; maps and builders use O(u) or O(n) extra space. For exact substring search, naive matching can be O(nm), so name KMP when worst-case linear matching matters. The key is a precise contract plus an invariant, not generic “string manipulation.”