def _mk_encode_tree(self):
"""Constructs the encoding tree form the heap containing _Node objects.
Returns the first node of the tree."""
freq_heap = MinHeap.from_iterable(self.freq_list)
while len(freq_heap) > 1:
# get the nodes with the smallest frequency
a = freq_heap.remove()
b = freq_heap.remove()
# make the new node and add it in it's proper position
new_node = TreeNode(a.freq + b.freq, content=None)
new_node.lchild = a
new_node.rchild = b
freq_heap.insert(new_node)
return freq_heap.remove()
