Count and Say
mediumTime: O(2^n)Space: O(2^n)
Signals to notice
describe previous term's digit groupsrun-length encodingiterative build
Brute force first
Not applicable — iterative IS the only approach.
The key insight
Start from '1'. Each step: scan for consecutive same digits, output count + digit. O(n × length).
What must stay true
Each term is the run-length encoding of the previous term. The transformation is deterministic.
Easy way to go wrong
Forgetting the last digit group — output it after the loop ends.