814. 二叉树剪枝

采用后序遍历的方式实现剪枝
class Solution(object):

def pruneTree(self, root):
    """
    :type root: TreeNode
    :rtype: TreeNode
    """
    if not root: return
    root.left = self.pruneTree(root.left)
    root.right = self.pruneTree(root.right)
    if not root.left and not root.right and root.val == 0:
        return None
    return root·