Program to reverse the directed graph in Python

Suppose we have a directed graph, we have to find its reverse so if an edge goes from u to v, it now goes from v to u. Here input will be an adjacency list, and if there are n nodes, the nodes will be (0, 1, ..., n-1).

So, if the input is like

then the output will be

To solve this, we will follow these steps −

• ans := a list of n different lists, where n is number of vertices
• for each index i, and adjacent list l in graph, do
  • for each x in l, do
    • insert i at the end of ans[x]
• return ans

Let us see the following implementation to get better understanding −

Example

class Solution:
   def solve(self, graph):
      ans = [[] for _ in graph]
      for i, l in enumerate(graph):
         for x in l:
            ans[x].append(i)
      return ans
ob = Solution()
graph = [[1,2],[4],[4],[1,2],[3]]
print(ob.solve(graph))

Input

[[1,2],[4],[4],[1,2],[3]]

Output

[[], [0, 3], [0, 3], [4], [1, 2]]