Base case (node is None) -> return 0

Return sum of:

- sumOfLeftLeaves of left node
- sumOfLeftLeaves of right node
- left node value if it is a leaf

```
class Solution(object):
def sumOfLeftLeaves(self, root):
if not root:
return 0
sum = self.sumOfLeftLeaves(root.left) + self.sumOfLeftLeaves(root.right)
if root.left and not root.left.left and not root.left.right:
sum += root.left.val
return sum
```