fix: use the same solution format for subset-sum as for coin-change

lib/bill-math.js

@@ -116,14 +116,8 @@ const getSolution_old = (units, amount, mode) => {
   
   const solver = sumService.subsetSum(billList, amount.toNumber())
   const solution = _.countBy(Math.floor, solver.next().value)
-  return _.reduce(
-    (acc, value) => {
-      acc.push({ denomination: _.toNumber(value), provisioned: solution[value] })
-      return acc
-    },
-    [],
-    _.keys(solution)
-  )
+  return Object.entries(solution)
+    .map(([denomination, provisioned]) => [_.toNumber(denomination), provisioned])
 }
 
 const getSolution = (units, amount) => {