Question

I am using an STL queue to implement a BFS (breadth first search) on a graph. I need to push a node in the queue if that node already doesn’t exist in the queue. However, STL queue does not allow iteration through its elements and hence I cannot use the STL find function.

I could use a flag for each node to mark them when they are visited and push them only when the flag is false, however, I need to run BFS multiple times and after each time I will have to reset all the flags, so I ended up using a counter instead of a flag, but I still would like to know if there is a standard way of finding an item in a queue.

Answer 1

I assume you’re implementing the concept of a “closed set” in your BFS? The standard way of doing that is to simply maintain a separate std::set or std::unordered_set of elements already encountered. That way, you get O(lg n) or O(1) lookup, while iterating through a queue, if it were supported, would take O(n) time.