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_topo.py

@@ -1,32 +0,0 @@
-# Kahn's Algorithm is used to find Topological ordering of Directed Acyclic Graph using BFS
-def topologicalSort(l):
-    indegree = [0] * len(l)
-    queue = []
-    topo = []
-    cnt = 0
-
-    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)
-        cnt += 1
-        topo.append(vertex)
-        for x in l[vertex]:
-            indegree[x] -= 1
-            if indegree[x] == 0:
-                queue.append(x)
-
-    if cnt != len(l):
-        print("Cycle exists")
-    else:
-        print(topo)
-
-# Adjacency List of Graph
-l = {0:[1,2], 1:[3], 2:[3], 3:[4,5], 4:[], 5:[]}
-topologicalSort(l)