Well, the idea is simple, and I write three functions to make this pretty clear, just for your reference :-)

```
# 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
"""
self.ans=0
self.findleftleaves(root)
return self.ans
def findleftleaves(self,root):
if not root:
return
if root.left and self.isLeaves(root.left):
self.ans+=root.left.val
self.findleftleaves(root.left)
self.findleftleaves(root.right)
return
def isLeaves(self,root):
return root.left==None and root.right==None
```