leetcode404 左叶子之和

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def sumOfLeftLeaves(self, root):
        """
        判断左叶子节点是难点
        """
        res = 0
        if not root:
            return 0
        if  root.left and not root.left.left and not root.left.right :###判断左叶子节点是难点
            res += root.left.val
        return res + self.sumOfLeftLeaves(root.left) + self.sumOfLeftLeaves(root.right)