Using DFS to traversal the tree and if the node is left leaf, add the value to result.

```
class Solution(object):
def sumOfLeftLeaves(self, root):
result = [0]
self.dfs(root, result, False)
return result[0]
# Use left to mark whether the current node is left child
def dfs(self, root, result, left):
if root is None:
return
else:
# If the node is left leaf, add the value to the result
if root.left is None and root.right is None and left:
result[0] += root.val
else:
self.dfs(root.left, result, True)
self.dfs(root.right, result, False)
return
```