This is another coin change problem tutorial we recommend. This one is by Cracking the Coding Interview (the bible of coding interview) author Gayle Laakmann Mcdowell. The explanation is crystal clear and very helpful. The only thing it didn't cover in depth is the idea of using a coin (reduce sum) and withholding a coin (not reducing the sum) while increasing the index to process the next coin. This is also a memoization and dynamic programming problem.
Previously we recommended Coin Change problem by O'Neill Code. That one uses a bottom up greedy approach to solve for smaller denominations. Then add the ways up as approaching bigger denominations. http://www.siliconvanity.com/2017/11/coin-change-problem-dyanmic-programming.html
Visualize using a coin versus not using a coin
|Gayle Laakmann Mcdowell helps visualize using a coin vs withholding a coin.
Visualize the need for memoization
|Hacker Rank tutorial video coin change problem