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.

解题思路：

• 首先明确左叶子的定义，即当前结点的左子树非空，且左子树是一个叶子，用代码表示为 root.left != None and root.left.left == None and root.left.right == None
• 当在左子树上找到左叶子后（比如例子中的 9），还需要在右子树上找到左叶子之和（递归），并且相加。用代码表示为 root.left.val + self.sumOfLeftLeaves(root.right)
• 如果当前结点的左子树不是叶子，则递归求左右子树的左子树之和。用代码表示为 self.sumOfLeftLeaves(root.left) + self.sumOfLeftLeaves(root.right)

Python 实现：

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def sumOfLeftLeaves(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root == None:
            return 0
        if root.left != None and root.left.left == None and root.left.right == None:
            return root.left.val + self.sumOfLeftLeaves(root.right) # 左叶子加上在右子树中求左叶子之和
        return self.sumOfLeftLeaves(root.left) + self.sumOfLeftLeaves(root.right) # 求左右子树中左叶子之和