The aim of this puzzle is, given a list of couples mentor / person influenced, to compute the number of people involved in the longest succession of influences. This can be done by building a graph where an edge going from a vertex x to a vertex y represents the influence of person x on person y. Then the answer is the length of the longest path in the graph +1 (its diameter +1).

## Python

Another example of nice OOP in Python. Instead of computing the diameter at the end, we keep updated for each vertex its depth, i.e its distance to its most ancient parent.

class Person(object):
'''Represents a person, which has a unique id, a depth in the tree of relations
and a list of persons she influenced'''
def __init__(self, id):
self.id, self.depth, self.influenced = id, 1, []
def add(self, p):
'''add person p amongst the person influenced'''
self.influenced.append(p)
def updateDepths(self):
'''update depths of the persons influenced, according to self depth'''
for p in self.influenced:
if (p.depth < self.depth+1):
p.depth = self.depth+1
p.updateDepths()
# number of relations
N = int(input())
persons = {}
for i in range(N):
x, y = [int(j) for j in input().split()]
if not x in persons:
persons[x] = Person(x)
if not y in persons:
persons[y] = Person(y)
px, py = persons[x], persons[y]
px.add(py)
px.updateDepths()
maximum = 0
for p in persons.values():
maximum = max(p.depth, maximum)
print(maximum)