在二叉树中找两个节点的最近公共祖先

public int lowestCommonAncestor (TreeNode root, int o1, int o2) {
        // write code here
        TreeNode res = dfs(root,o1,o2);
        if (res==null) return -1;
        return res.val;
    }

    private TreeNode dfs(TreeNode root, int o1, int o2) {
        if (root==null) return null;
        if (root.val==o1 || root.val==o2) return root;
        TreeNode left = dfs(root.left,o1,o2);
        TreeNode right = dfs(root.right,o1,o2);
        if (left!=null && right!=null) return root;
        if (left==null) return right;
        return left;
    }