新模版：LCA问题

class Solution {
    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);

        if (left == null && right == null){
            return null;
        }

        else if (left != null && right != null){
            return root;
        }

        else return left == null?right : left;
    }
}