Find Lowest Common Ancestor in a Binary Tree

Statement

Given a binary tree and two of its nodes, find their lowest common ancestor (LCA).

Implement the function lowestCommonAncestor(root, node1, node2), where root is the root node of the binary tree. The function should return the value of the LCA node.

Example

Consider the following binary tree as an example, where $20$ is the LCA of $15$ and $7$: