symmetric order for binary tree

例句：

1. 1.The two sides of the coin are symmetrical.
   硬币的两面是对称的。
2. 2.The arrangement is symmetrical, with each side mirroring the other.
   这种布局是对称的，每一侧都与另一侧相对应。
3. 3. The design is symmetrical, creating a balanced look.
   这个设计是对称的，给人一种平衡感。