Candy Crush - Algorithm

The algorithm has three phases per round:

$1.$ Mark:: Find all horizontal and vertical runs of $3$+ same candies. Mark them for removal.

$2.$ Crush:: Remove all marked candies (set to $0$ or empty).

$3.$ Drop:: Apply gravity. Each column's candies fall to fill empty spaces. Repeat until no candies are marked. The greedy aspect: crush everything possible before dropping. Common bug: crushing one match at a time instead of all at once can miss cascading matches. Time: $O(n)$. Space: $O(n)$.