How to create binary edge graph in pydot?

Question

I am having a list of values as lst=[5,7,3,1]. I want to create a graph with each node must have max of 2 edges. One left node should be in left side to root which has less than to root node value as well as one right node should be in right side to root which has higher value to root node value. I used the following code as

import pydot
lst= [5,7,3,1]
lst_len=len(lst)
graph = pydot.Dot(graph_type='graph')

for i in lst:
  start=lst.index(i)
  if start < lst_len-1:
    end=start+1
  edge = pydot.Edge(lst[start], lst[end])
  graph.add_edge(edge)
graph.write_png('bst.png')

I got the output as

But I want the output as

How can I modify my code? Guide me.. thanks..