DSA - 3 new - DP, Hashmap

213. House Robber II

Insight: Same as House Robber (iterative DP). Since first & last are adjacent, split into 2 cases:

• Rob from index 0 → n-2
• Rob from index 1 → n-1
  Take max of both.

Comment: Brain fog. Couldn’t even recall/formulate base House Robber logic.

217. Contains Duplicate

Insight:

1. Convert to Set → if size reduces → duplicate exists
2. Use HashMap → check while inserting

Comment: Straightforward. Can do in O(n) using set/hash.

91. Decode Ways

Insight: DFS + memoization
Recurrence: dfs(i) = dfs(i+1) + dfs(i+2)
Base cases:

• Out of bounds → 1
• Leading ‘0’ → 0
• Two-digit valid only if <= 26

Comment: Weak formulation. Took help (NeetCode + Gemini) to complete.