```
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def sumOfLeftLeaves(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if not root:
return 0
stack ,res = [root] , 0
while stack:
node = stack.pop()
if node.left and not node.left.left and not node.left.right:
res += (node.left.val)
if node.left:
stack += [node.left]
if node.right:
stack += [node.right]
return res
```