lowestCommonAncestorOfABinaryTree.java

/*
  Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

  According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

  Given the following binary tree:  root = [3,5,1,6,2,0,8,null,null,7,4]

  e.g 1
  Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
  Output: 3
  Explanation: The LCA of nodes 5 and 1 is 3.

  e.g 2
  Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
  Output: 5
  Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

  https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/
*/


public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    if (root == null || root == p || root == q) return root;
    TreeNode left = lowestCommonAncestor(root.left, p, q);
    TreeNode right = lowestCommonAncestor(root.right, p, q);
    return left == null ? right : right == null ? left : root;
}