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LeetCode 2471 Minimum Number of Operations to Sort a Binary Tree by Level uses BFS plus minimum-swaps-to-sort-an-array. Solve in O(n log n) — a popular Google and Meta interview question.
LeetCode 173 BST Iterator implements a controlled inorder traversal with O(h) memory and amortized O(1) next. Asked at Amazon, Google, Meta, and Apple as a class-design tree question.
LeetCode 666 Path Sum IV reconstructs a tree from depth-position-value encoded integers and returns the sum of all root-to-leaf paths. Solve with hashmap DFS in O(n) — frequent at Amazon and Alibaba.
LeetCode 272 (Hard) frequently asked at Google and Meta. Use BST inorder traversal to get a sorted list, then a two-pointer shrink window selects the k closest values to a target in O(n) time.
LeetCode 156 (Medium) classically asked at Google and LinkedIn. Re-root a binary tree by flipping each left child into the new parent and the original parent into the new right child, in O(n) time and O(1) extra space iteratively.