Social Network

How would you design the data structures for a very large social network like Facebook or LinkedIn? Describe how you would design an algorithm to show the shortest path between two people (e.g., Me -> Bob -> Susan -> Jason -> You)


construct a graph by treating each person as a node and letting an edge between two nodes indicate that the two users are friends.

BFS search -> bidirectional breadth-first-search

Suppose every person has k friends, and node S and node D have a friend C in common.

  • Traditional bfs from S to D: go through k+k*k nodes
  • Bidirectional bfs: go through 2k nodes

Generalizing this to a path of length q

  • BFS: O(qk)
  • Bidirectional BFS: O(qk/2 + qk/2)

Handle the Millions of User

data in different machines, use hashtable or other mechanism to index them and search them for the same time.


  • Reduce machine jumps
  • Smart division of people and machines(related data should be put together)
  • Instead of marking VISITED, we use a hashtable to record the nodes we have already visited