Moving

Let $e$ be the right endpoint reachable with the current number of moves, and let $f$ be the farthest position reachable with one additional move from this range.

Scan positions from left to right and update $f=\max(f,i+A_i)$. When $i=e$, the computed $f$ becomes the endpoint for the next move count. If the range no longer expands, position $N$ is unreachable.

The time complexity is $O(N)$ per test case.

Solution written by GPT5