python 通过函数无返回的修改数据结构，在函数外部，数据结构也可被修改

class TreeNode:
def __init__(self,x):
self.val = x
self.left = None
self.right = None

def change(root):
root.left,root.right = root.right,root.left

if __name__=="__main__":
root = TreeNode(0)
root.left = TreeNode(1)
root.right = TreeNode(2)

change(root)
print root.left.val