1: link every left child to right child

2: link every right child to the left child of his father's next

```
def connect(self, root):
if root==None:
return
def connect1(r):
if r.left: r.left.next=r.right
if r.right and r.next: r.right.next=r.next.left
if r.left and r.right:
connect1(r.left)
connect1(r.right)
connect1(root)
```