Idea is very simple. Just traverse the whole tree using stack. If the left child is leave, add it to the result.

```
class Solution(object):
def sumOfLeftLeaves(self, root):
if not root: return 0
s = [root]
res = 0
while s:
tmp = s.pop()
if tmp.left:
s.append(tmp.left)
if not tmp.left.left and not tmp.left.right:
res += tmp.left.val
if tmp.right:
s.append(tmp.right)
return res
```