class Solution:
# @param root, a tree node
# @return nothing
def connect(self, root):
stack=[]
if root :
stack.append(root)
while stack:
popping=[]
while stack:
node=stack.pop(0)
if stack:
node.next=stack[0]
if node.left:
popping.append(node.left)
if node.right:
popping.append(node.right)
stack=popping
return root