16. leetcode 404. Sum of Left Leaves

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。