Write a Python function that returns the distance between two 
nodes in a tree that have no parent and at least one leaf.

def distance(i, j, D, leaves):
    
    #1. Obtain leaves of i in a list leaves_i
    #2. Obtain leaves of j in a list leaves_j
    #3. Initialize dist = 0
    #4. For each leaf x in leaves_i
    #      For each leaf y in leaves_j
    #          Add D[int(x)][int(y)] to dist
    #5. Divide dist by len(leaves_i)*len(leaves_j)
    #6. Return dist

Submit a hardcopy on Apr 19th in class.