Add topological sorter

Author: RoundofThree

README.md

@@ -4,3 +4,5 @@
 | -- | -- |
 |[Signature parser](create_electronic_sign) | Parse handwritten signature in JPG/JPEG/PNG to a new JPG with black signature and transparent background. |
 |[2-opt TSP](2-opt-tsp) | Find local minimum weighted Hamiltonian cycle in a complete graph expressed with a cost adjacency matrix. |
+|[Topological sorter](topological_sorter) | From a directed graph, find a topologically ordered list of vertices. | 
+|[Connected components graph](scc-graph) | From a directed graph, return a graph with sets as vertices. |

topological_sorter/README.md

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+## Topological sorter 
+
+Input: a directed graph
+```
+Enter an edge, eg.: A->B
+a->b
+Enter an edge, eg.: A->B
+b->c
+Enter an edge, eg.: A->B
+j->a
+Enter an edge, eg.: A->B
+ # blank line to finish 
+``` 
+
+Output: an ordered list 
+```
+Directed graph constructed.                                                                                            
+Topologically sorted:  ['j', 'a', 'b', 'c'] 
+```

topological_sorter/topological_sorter.py

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+from collections import defaultdict
+from typing import DefaultDict
+
+def topological_sort(graph, node):
+    ret = []
+    visited = set()
+
+    def rec(node):
+        visited.add(node)
+        for neighbour in graph[node]:
+            if neighbour not in visited:
+                visited.add(neighbour)
+                rec(neighbour)
+        ret.insert(0, node) 
+    # check all keys in graph are visited 
+    for k in list(graph):
+        if k not in visited:
+            rec(k)
+    return ret
+
+if __name__ == '__main__':
+    graph = defaultdict(set)  # string to set of strings 
+    while True:
+        edge = input("Enter an edge, eg.: A->B \n").strip().split("->")
+        if len(edge) == 2:
+            graph[edge[0]].add(edge[1])
+        else:
+            print("Directed graph constructed.")
+            break 
+    print("Topologically sorted: ", topological_sort(graph, next(iter(graph))))