Hot100: Symmetric Tree (Mirror Recursion / BFS ACERS Guide)
Subtitle / Summary The hard part of Symmetric Tree is not traversal itself, but comparison direction. You are not comparing left to left and right to right. You are comparing mirrored positions. This ACERS guide explains the mirror-recursion contract, the BFS queue-of-pairs variant, and real engineering cases where symmetry checking matters. Reading time: 10-12 min Tags: Hot100, binary tree, DFS, BFS, symmetry SEO keywords: Hot100, Symmetric Tree, mirror recursion, binary tree symmetry, BFS, LeetCode 101 Meta description: A systematic guide to LeetCode 101 from mirror recursion to pairwise BFS symmetry checks, with engineering scenarios and runnable multi-language implementations. Target Readers Hot100 learners moving from Same Tree to mirror comparison Developers who can write ordinary tree recursion but still mix up outside and inside pairs Engineers who need left-right symmetry validation for layouts, topology templates, or mirrored structures Background / Motivation LeetCode 101 is excellent training for directional thinking in tree problems: ...