Lecture 9. Dynamic Programming: optimal coin change We have seen this problem before: you are given an amount in cents, and you want to make change using a system of denominations, using the smallest number of coins possible. Sometimes the greedy algorithm gives the optimal solution. But sometimes (as we have seen) it does not -- an example was the system (12, 5, 1), where the greedy algorithm gives 15 = but a better answer is 15 = And sometimes Greedy cannot even find the proper changes: (25,10,4), change for 41 cents. So how can we find the optimal solution (fewest coins) when the greedy algorithm may not work? One way is using dynamic programming.

Optimal coin-changing -- The idea: to make change for n cents, the optimal method must use some denomination d i. That is, the optimal is made by choosing the optimal solution for n – d i for some d i, and adding one coin of d i to it. We don't know which d i to use, but some must work. So we try them all, assuming we know how to make optimal changes for < n cents. To formalize: letting opt[j] be the optimal number of coins to make change for j cents, we have: opt[j] = 1 + min( opt[j-d i ]) over all d i ≤j

Optimal coin-change: coins(n, d[1..k]) /* returns optimal number of coins to make change for n using denominations d[1..k], with d[1] = 1 */ for j := 1 to n do opt[j] := infinity; for i := k downto 1 do if d[i] = j then opt[j] := 1; largest[j] := j; else if d[i] < j then a := 1+opt[j-d[i]]; if a < opt[j] then opt[j] := a; largest[j] := d[i]; return(opt[n]) Running time: O(nk) Input size: ?

Example. Suppose we use the system of denominations (1,5,18,25). To represent 29, the greedy algorithm gives , Our algorithm: largest coin =11, which has a representation of size 3, with largest coin =6, which has a representation of size 2, with largest coin =1. So 29 = j opt[j] largest[j] j opt[j] largest[j]

Paradigm #7. Exploiting the problems’ structure Example: Computing GCD We say d is a divisor of u if there exists an integer k such that u = kd. We write d | u. We say d is a common divisor of u and v if d | u and d | v. We say d is the greatest common divisor if d is a common divisor of u and v and no other common divisor is larger. Example: gcd(12,18)=6. Observe: d | u & d | v iff d | v and d | (u mod v) Euclid’s Alg (300BC) Euclid(u,v) while (v ≠ 0) do (u, v) := (v, u mod v) return(u) Thm (Finck 1841). This algorithm runs in O(log v) steps, for u>v>0. Proof: Let r = u mod v. Case 1. v>u/2, then r =u-v < u/2 Case 2. v=u/2, then r=0 Case 3. v< u/2, then r<v<u/2 So after 2 steps, the numbers decrease by factor of 2. Hence after O(log v) steps, to 0.