Numbers That Only Climb — Solution & Editorial

Back to the Problem

Digit DP with prefix subtraction f(R) − f(L−1): the state is the previous digit; the next digit must be ≥ it. Carry a 'tight' flag for the upper bound and a 'leading-zero' flag.

Complexity: O(digits · states)

Watch out: After leading zeros, the first real digit starts a fresh comparison; keeping previous=0 through leading zeros happens to work here since any first digit >= 0.