leetcode404.左叶子之和

 加一个is_left判定，很好理解的~~

class Solution:
    def sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
        self.count_sum = 0
        def traversal(node, is_left):
            if not node:
                return
            if not node.left and not node.right and is_left:
                self.count_sum += node.val
            traversal(node.left, True)
            traversal(node.right, False)
        traversal(root, False)
        return self.count_sum