Implement Trie | One Walk, Three Operations | LeetCode 75 #70

Implement Trie (Prefix Tree) — LeetCode 208 | One Walk, Three Operations The trie onboarding video: build the node-structure intuition (children array + isEnd flag) and watch insert, search and startsWith as ONE walk down the tree with three different endings. What you will learn: Why a trie: hashset gives fast search but NO prefix queries; sorted list gives prefix range but costs O(L log n) plus O(n) inserts The TrieNode: children[26] indexed by c - 'a', and the 208-bytes-per-node memory tradeoff vs map The isEnd flag: why search("app") is FALSE when only "apple" was inserted — the classic interview trap insert / search / startsWith as the same walk, different endings Complexity: O(L) time per operation, O(total characters) space C++ code explained line by line, debugger-style, each line lighting up as it mutates the live tree. Chapters: 0:00:00 Hook: one walk, three endings 0:01:29 The problem (LC 208) 0:03:14 HARD 1: why a trie, not a hashset 0:04:57 HARD 1: prefix is a location 0:06:35 HARD 2: children[26] and the memory fee 0:09:33 HARD 3: the isEnd flag trap 0:12:14 Dry run: insert apple 0:13:52 Dry run: search + startsWith 0:15:41 Dry run: insert app, ape (sharing) 0:17:14 Dry run: startsWith walk 0:18:53 Dry run 2: bat, ball, bad 0:20:33 Code: struct TrieNode 0:22:32 Code: insert (live encore) 0:24:44 Code: search 0:26:21 Code: startsWith 0:28:05 Proof: O(L) time 0:30:41 Edge cases 0:32:36 Recap + next video LeetCode 75 series — video 70. Full C++ solution in the pinned comment.