Monte Carlo tree searchfeatured
Observable-state MCTS
Grow the look-ahead by simulation, but only ever let the search know what a player would actually be able to see.
rejectedrecordedGrow the look-ahead only where it seems promising, guided by quick simulated playouts.
Instead of examining every branch to a fixed depth, spend each unit of thinking where past playouts say it is most worth spending, and grow the tree lopsided.
Read the primer3 approaches, featured pages first. Each card is one theory of how to choose a column and what happened when it was tried.