404. 左叶子之和

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# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
        if root==None:  return 0
        if root.left==None and root.right==None: return 0
        # 后序遍历
        left_value = self.sumOfLeftLeaves(root.left)
        right_value = self.sumOfLeftLeaves(root.right)
        if root.left!=None and root.left.left==None and root.left.right==None:
            left_value = root.left.val 
        left_sum = left_value + right_value
        return left_sum