题目：计算给定二叉树的所有左叶子之和。
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3
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9 20
/
15 7
在这个二叉树中，有两个左叶子，分别是 9 和 15，所以返回 24
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思路：必须是叶子节点(是子节点且此子节点的左右孩子都是空)，才能append
代码：
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class :
def __init__(self, x):
self.val = x
self.left = None
self.right = None
class Solution:
def helper(self, root,res):
if root and root.left:
if not root.left.left and not root.left.right:
res.append(root.left.val)
self.helper(root.left,res)
if root and root.right:
self.helper(root.right,res)
def sumOfLeftLeaves(self, root):
"""
:type root: TreeNode
:rtype: int
"""
res = []
self.helper(root,res)
return sum(res)
tree = TreeNode(3)
left_tree = TreeNode(9)
right_tree = TreeNode(20)
right_tree.left = TreeNode(15)
right_tree.right = TreeNode(7)
tree.left = left_tree
tree.right = right_tree
print(Solution().sumOfLeftLeaves(TreeNode(None)))
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