Coin Change DP

Why it matters

Coin change is classic DP. Understanding shapes unbounded knapsack.

The architecture

dp[a] = min coins for amount a.

Or dp[a] = number of ways.

Iterate over amounts + coins.

How it works end to end

Min coins: dp[a] = min(dp[a], dp[a-c] + 1).

Ways: dp[a] += dp[a-c] with coin outer loop.

Unbounded (repeats OK).