int res = 1;

    public int diameterOfBinaryTree(TreeNode root) {

        helper(root);

        return res-1;

    }

    public int[] helper(TreeNode root)

    {

        //两个量分别是节点深度，节点最大diameter

        if (root==null) return new int[]{0,0};

        int cur = 0;

        int[] l = helper(root.left);

        int[] r = helper(root.right);

        cur = Math.max(Math.max(l[1],r[1]),l[0]+r[0]+1);

        res = Math.max(res,cur);

        return new int[]{Math.max(l[0],r[0])+1,cur};

    }