Organize graph algorithms (#719)

* organized graph algorithms

* all graph algorithms in Graphs/ folder

* all graph algorithms are in one folder

* Rename number theory/factorial_python.py to maths/factorial_python.py

graphs/kahns_algorithm_long.py

@@ -1,30 +0,0 @@
-# Finding longest distance in Directed Acyclic Graph using KahnsAlgorithm
-def longestDistance(l):
-    indegree = [0] * len(l)
-    queue = []
-    longDist = [1] * len(l)
-
-    for key, values in l.items():
-        for i in values:
-            indegree[i] += 1
-
-    for i in range(len(indegree)):
-        if indegree[i] == 0:
-            queue.append(i)
-
-    while(queue):
-        vertex = queue.pop(0)
-        for x in l[vertex]:
-            indegree[x] -= 1
-
-            if longDist[vertex] + 1 > longDist[x]:
-                longDist[x] =  longDist[vertex] + 1
-
-            if indegree[x] == 0:
-                queue.append(x)
-
-    print(max(longDist))
-
-# Adjacency list of Graph
-l = {0:[2,3,4], 1:[2,7], 2:[5], 3:[5,7], 4:[7], 5:[6], 6:[7], 7:[]}
-longestDistance(l)