Sum of Left Leaves (Leetcode 404)

Problem Link: https://leetcode.com/problems/sum-of-left-leaves/

class Solution:
    
    def sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
        
        global ans
        ans=0
        
        def fun(root):
            global ans
            if(root is None):
                return 0

            if(root.left is not None):
                if(root.left.left is None and root.left.right is None):
                    ans=ans+root.left.val

            fun(root.left)
            fun(root.right)
            return ans
        
        return fun(root)