# Coin Change

Coin Change is the problem of finding the number of ways of making changes for a particular amount of cents, n, using a given set of denominations $d_1 \ldots d_m$. It is a general case of Integer Partition, and can be solved with dynamic programming. (The Min-Coin Change is a common variation of this problem.)

##  Overview

The problem is typically asked as: If we want to make change for N cents, and we have infinite supply of each of $S = \{ S_1, S_2, \ldots, S_m \}$ valued coins, how many ways can we make the change? (For simplicity's sake, the order does not matter.)

It is more precisely defined as:

Given an integer N and a set of integers $S = \{ S_1, S_2, \ldots, S_m \}$, how many ways can one express N as a linear combination of $S = \{ S_1, S_2, \ldots, S_m \}$ with non-negative coefficients?

Mathematically, how many solutions are there to $N = \sum_{k = 1 \ldots m}{ x_k S_k }$ where $x_k \geq 0, k \in \{ 1 \ldots m \}$

For example, for N = 4,S = {1,2,3}, there are four solutions: {1,1,1,1},{1,1,2},{2,2},{1,3}.

Other common variations on the problem include decision-based question, such as:

Is there a solution for $N = \sum_{k = 1 \ldots m}{ x_k S_k }$ where $x_k \geq 0, k \in \{ 1 \ldots m \}$ (Is there a solution for integer N and a set of integers $S = \{ S_1, S_2, \ldots, S_m \}$?)

Is there a solution for $N = \sum_{k = 1 \ldots m}{ x_k S_k }$ where $x_k \geq 0, k \in \{ 1 \ldots m \}, \sum_{k = 1 \ldots m}{x_k} \leq T$ (Is there a solution for integer N and a set of integers $S = \{ S_1, S_2, \ldots, S_m \}$ such that $\sum_{k = 1 \ldots m}{x_k} \leq T$ - using at most T coins)

##  Recursive Formulation

We are trying to count the number of distinct sets.

Since order does not matter, we will impose that our solutions (sets) are all sorted in non-decreasing order (Thus, we are looking at sorted-set solutions: collections).

For a particular N and $S = \{ S_1, S_2, \ldots, S_m \}$ (now with the restriction that $S_1 < S_2 < \ldots < S_m$, our solutions can be constructed in non-decreasing order), the set of solutions for this problem, C(N,m), can be partitioned into two sets:

• There are those sets that do not contain any Sm and
• Those sets that contain at least 1 Sm

If a solution does not contain Sm, then we can solve the subproblem of N with $S = \{ S_1, S_2, \ldots, S_{m-1} \}$, or the solutions of C(N,m - 1).

If a solution does contain Sm, then we are using at least one Sm, thus we are now solving the subproblem of N - Sm, $S = \{ S_1, S_2, \ldots, S_m \}$. This is C(N - Sm,m).

Thus, we can formulate the following:

C(N,m) = C(N,m - 1) + C(N - Sm,m)

with the base cases:

• C(N,m) = 1,N = 0 (one solution -- we have no money, exactly one way to solve the problem - by choosing no coin change, or, more precisely, to choose coin change of 0)
• $C( N, m ) = 0, N \leq 0$ (no solution -- negative sum of money)
• $C( N, m ) = 0, N \geq 1, m \leq 0$ (no solution -- we have money, but no change available)

###  Pseudocode

def count( n, m ):
if n == 0:
return 1
if n < 0:
return 0
if m <= 0 and n >= 1: #m < 0 for zero indexed programming languages
return 0

return count( n, m - 1 ) + count( n - S[m], m )

##  Dynamic Programming

Note that the recursion satisfies the weak ordering $R(n,m) < R(x,y) \iff n \leq x, m \leq y, (n,m) \ne (x,y)$. As a result, this satisfies the optimal-substructure property of dynamic programming.

The result can be computed in O(nm) time - the above pseudocode can easily be modified to contain memoization. It can be also rewritten as:

func count( n, m )

for i from 0 to n
for j from 0 to m
if i equals 0
table[i,j] = 1
else if j equals 0
table [i,j] = 0
else if S_j greater than i
table[ i, j ] = table[ i, j - 1 ]
else
table[ i, j ] = table[ i - S_j, j ] + table[ i, j - 1 ]

return table[ n, m ]