Section Recap

You've learned digit DP. You can now:

$1.$ Count numbers in huge ranges without brute force.

$2.$ Handle tight bounds to stay within limits.

$3.$ Track used digits with bitmasks.

$4.$ Apply digit DP in binary for bit-based constraints. The observation: build numbers digit by digit, tracking just enough state to enforce your constraints. This turns impossible $O(N)$ problems into easy $O(\log N \times states)$ solutions. For training, solve the problems here's problemset. Then move on to Game Theory DP where you'll handle two-player games.