Coin Change: greedy, the overflow bug, call counts, loop order
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1) Greedy fails: coins {1,3,4}, amount 6
   greedy      -> 3 coins (4+1+1)
   DP (table)  -> 2 coins (3+3)

2) 'Impossible' = Integer.MAX_VALUE, then dp[a-c] + 1: coins {2}, amount 3 (really impossible)
   correct (impossible = amount + 1) -> -1
   Integer.MAX_VALUE version         -> -2147483648

3) Calls made: coins {1,2,5}, amount 27, answer 6
   naive recursion -> 2688332 calls
   memoized        -> 77 calls

4) Loop order changes the question: coins {1,2,5}, amount 5
   coins in the OUTER loop  -> 4 (combinations: 5 / 2+2+1 / 2+1+1+1 / 1x5)
   amount in the OUTER loop -> 9 (permutations: 1+2 and 2+1 now count separately)