```python # 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: self.left_sum = 0 def cal_left_sum( r ): if not r: return # 站在r上 r的左节点是叶子节点 if r.left and not r.left.left and not r.left .right: self.left_sum += r.left.val cal_left_sum( r.left ) cal_left_sum( r.right ) cal_left_sum( root ) return self.left_sum ``` ```python # 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 not root: return 0 if not root.left and not root.right: return 0 l = self.sumOfLeftLeaves( root.left ) if root.left and not root.left.left and not root.left.right: l += root.left.val r = self.sumOfLeftLeaves( root.right ) return l + r ```