Select random element in an unordered_map

Question

I define an unordered_map like this:

std::unordered_map edges;

Is there a efficient way to choose a rando

Answer 1

Strict O(1) solution without buckets:

1. Keep a vector of keys, when you need to get a random element from your map, select a random key from the vector and return corresponding value from the map - takes constant time
2. If you insert a key-value pair into your map, check if such key is already present, and if it's not the case, add that key to your key vector - takes constant time
3. If you want to remove an element from the map after it was selected, swap the key you selected with the back() element of your key vector and call pop_back(), after that erase the element from the map and return the value - takes constant time

However, there is a limitation: if you want to delete elements from the map aside from random picking, you need to fix your key vector, this takes O(n) with naive approach. But still there is a way to get O(1) performance: keep a map that tells you where the key is in the key vector and update it with swap :)