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Tree•Pattern: Post-order DFS•Medium

Lowest Common Ancestor (LCA) of a binary tree

Problem Intuition & Objective

Find the deepest node that is an ancestor of both given nodes.

Time ComplexityO(n)
Optimal
Space ComplexityO(h)
Memory Bounds
Optimal Algorithmic Approach

Post-order DFS: the first node where both subtrees return hits is the LCA.

Core Invariants & Edge Cases

  • If current node is p or q, return it
  • Recurse left and right; if both return non-null, current node is LCA
  • Otherwise return the non-null result
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