Alpha-Beta Pruning

• When you are going forward with minimax, you want to avoid as many branches as possible.
• Why?
• You can use pruning and this won't adversely effect what is happening.
• Minimax(root) = max(min(3,12,8),min(2,x,y), min (1,z,w))= 3.
• The key here is that I don't need to expand x, y, z and w.
• No matter what happens, the current ply will never choose those branches.
• This of course works the other way too.
• Minimax(root) = min(max(3,12,8),max(20,x,y),max(13,z,w)) = 12
• In the average case, you can prune half the branches; this might get you an extra ply.