There is a one-dimensional garden of length n - 1 meters. Every 1 meter there is a sprinkler,
and each sprinkler has a different covering range. The sprinklers are labelled 0 ~ n - 1,
indicating that the sprinklers are 0 ~ n - 1 meters away from the start of the fountain.
The ith sprinkler covers r[i] meters of land, meaning that everywhere from
i - r[i] meters to i + r[i] meters (inclusive) will be covered by the sprinkler.
This is a companion discussion topic for the original entry at https://algo.monster/problems/twitter_oa_activate_fountain/