題目：
Given two binary trees, write a function to check if they are equal or not.
Two binary trees are considered equal if they are structurally identical and the nodes have the same value.
判斷兩棵二叉樹是否相等，即結構相同，且對應的節點具有相同的值
思路：由樹的前序、中序和後序遍歷，想到用遞歸的方法，先判斷根節點是否相同，再依次判斷左子樹和右子樹是否相同。
代碼AC：
