最近公共祖先

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