The change-making problem addresses the following question: how can a given amount of money be made with the least number of coins of given denominations? It is a knapsack type problem, and has applications wider than just currency.
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Given a set of integer coin values {w1, w2, ..., wn} where w1 = 1 and wj < wj+1 for 1 ≤ j ≤ n − 1, and a positive integer W, find a set of non-negative integers {x1, x2, ..., xn} which minimize
subject to
Making a list of ways to make each amount from one upwards (an example of a dynamic programming method)
Integer Linear Programming is often a quick way to solve this kind of problem, but the time it will take to resolve the problem is not certain, and may be slow in some cases
In the US (and most other) coin systems, a greedy algorithm of picking the largest denomination of coin which is not greater than the remaining amount to be made will always produce the optimal result. This is not automatically the case, though: if the coin denominations were 1, 3 and 4, then to make 6, the greedy algorithm would choose three coins (4,1,1) whereas the optimal solution is two coins (3,3).