The sum of an emptytree is zero, and the sum of other trees is the sum of the left tree plus the value of the Integer plus the sum of the right tree.
空树的汇总是零,其他树的汇总是左树加上Integer的值再加上右树的值。
3
After the class, small squirrel found a emptytree, he put some leaves and dry grass in it. Squirrel finished his home work, after that he went to monkey's home happily.