LeetCode Q 236 - Lowest Common Ancestor of a Binary Tree

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).

Example 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.
Example 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.

Note:

All of the nodes’ values will be unique.
p and q are different and both values will exist in the binary tree.

Solution: DFS

Explanation:

Code:

public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
  if (root == null) return null;
  if (p == null) return q;
  if (q == null) return p;
  return dfs(root, p, q);
}

private TreeNode dfs (TreeNode node, TreeNode p, TreeNode q) {

  if (node == null || node == p || node == q) return node;

  TreeNode l = dfs(node.left, p, q);
  TreeNode r = dfs(node.right, p, q);

  if (l == null) return r;
  if (r == null) return l;

  return node;
}

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