1d dp

House Robber is the tidiest choose-with-a-constraint 1D DP, and it reduces to O(1) space. Interviewers look for the take-this-or-skip recurrence and the two-variable rolling optimization.

Decode Ways is a 1D DP whose difficulty sits entirely in the edge cases: zeros and the valid 1-26 range. Interviewers look for the take-one-digit-or-two recurrence plus disciplined validity checks. The answer follows.