All values in the leftmost subtree will be less than a1, all values in the middle subtree will be between a1 and a2, and all values in the rightmost subtree will be greater than a2.
左边子树的所有值都必须小于 a1 ，中间子树的所有值都必须在 a1 和a2 之间，右边子树的所有值都必须大于 a2 。