How huffman tree uses MinHeap?
Building a huffman tree is bottom up. 


You start with the all the leave nodes with their frequency.
Then you select and remove the 2 nodes with the smallest frequencies
create and insert a new compound node with the 2 selected nodes and it's new frequency is the sum of the 2 nodes
if set has 2 or more nodes repeat from step 2


Selecting the 2 nodes with the least frequencies requires a min heap.