LeetCode 518 Coin Change II - Problem Statement

Given coins of different denominations and a total amount, return the number of combinations to make that amount. You can use each coin unlimited times. For example, with coins $[1, 2, 5]$ and amount $5$, there are $4$ ways: $[5], [2,2,1], [2,1,1,1], [1,1,1,1,1]$. This is unbounded knapsack with counting.

But there's a trap. Notice the problem says "combinations", not "permutations". Using $[1,2]$ and $[2,1]$ counts as one way, not two. If you're not careful, your DP will count permutations instead. The fix involves loop order, which trips up a lot of people.