LeetCode 464 Can I Win - Key Idea

State: which numbers have been used. With $n \leq 20$, a bitmask works perfectly. Bit $i$ set means number $i+1$ is used.

Winning condition: current player can pick a number that either wins immediately (total $\geq$ target) or leaves opponent in a losing state. We don't need to track whose turn it is. The current player is always "me" in the recursion. If I can win from this state, the state is winning.