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Shortest Palindrome (LC 214) is a hard FAANG problem that hides a classic KMP application. Concatenate s with reversed(s) and the failure function reveals the longest palindromic prefix in linear time. Master this trick and you unlock half a dozen related KMP applications.
Counting distinct substrings of a string is the gateway problem to suffix arrays and suffix automata. Three classical solutions — n^2 hash set, suffix array plus LCP, and suffix automaton — span the full toolbox of competitive programming and FAANG hard interviews.
String Compression (LC 443) is a deceptively careful two-pointer problem with strict in-place memory requirements. Master the read-write pointer pattern and you have the blueprint for in-place array transformations across many FAANG questions.
Beyond the basic trie there is a rich landscape of advanced applications — XOR tries for maximum-XOR queries, prefix-and-suffix tries, ternary tries, compressed tries (PATRICIA), and persistent tries. Master these and you have the toolkit for half a dozen FAANG hards.
A curated and battle-tested set of the string problems that show up most often in Meta and Google onsite loops, with the canonical pattern for each. Cover this list and you cover roughly 80 percent of string-heavy FAANG screens.