Find the sum of all left leaves in a given binary tree.
Example:
3
/ \
9 20
/ \
15 7
There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
思路：还是递归。仔细看代码，体会递归的设计方法。
转载于:https://www.cnblogs.com/vincent93/p/6686533.html
