```
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if (root == null || root == p || root == q)
// found p or q or touch the ground
return root;
// search p and q from left and right
TreeNode left = lowestCommonAncestor(root.left, p, q);
TreeNode right = lowestCommonAncestor(root.right, p, q);
if (left != null && right != null)
// from root's left & right we found both p and q, so root is the LCA
return root;
else
// left is not null means from left's left & right we found both q and q
// so left is the LCA, otherwise, right is the answer
return left != null ? left : right;
}
```