2015-day17: recursive backtrack to valid combinations

2015/day17/main.go

@@ -0,0 +1,67 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"math"
+	"strings"
+
+	"github.com/alexchao26/advent-of-code-go/cast"
+	"github.com/alexchao26/advent-of-code-go/mathutil"
+	"github.com/alexchao26/advent-of-code-go/util"
+)
+
+func main() {
+	var part int
+	flag.IntVar(&part, "part", 1, "part 1 or 2")
+	flag.Parse()
+	fmt.Println("Running part", part)
+
+	ans := eggnogCombinations(util.ReadFile("./input.txt"), 150, part)
+	fmt.Println("Output:", ans)
+}
+
+func eggnogCombinations(input string, target int, part int) int {
+	var nums []int
+	for _, line := range strings.Split(input, "\n") {
+		nums = append(nums, cast.ToInt(line))
+	}
+
+	allIndexCombinations := backtrack(nums, 0, target, []int{})
+	// part 1, just return len
+	if part == 1 {
+		return len(allIndexCombinations)
+	}
+
+	// part 2, get the number of combinations w/ the lowest length
+	minLen := math.MaxInt32
+	for _, comb := range allIndexCombinations {
+		minLen = mathutil.MinInt(minLen, len(comb))
+	}
+
+	var count int
+	for _, comb := range allIndexCombinations {
+		if len(comb) == minLen {
+			count++
+		}
+	}
+
+	return count
+}
+
+func backtrack(nums []int, startingIndex, remaining int, usedIndices []int) [][]int {
+	if remaining == 0 {
+		return [][]int{append([]int{}, usedIndices...)}
+	}
+	if remaining < 0 {
+		return nil
+	}
+
+	var validReturns [][]int
+	for i := startingIndex; i < len(nums); i++ {
+		usedIndices = append(usedIndices, i)
+		validReturns = append(validReturns, backtrack(nums, i+1, remaining-nums[i], usedIndices)...)
+		usedIndices = usedIndices[:len(usedIndices)-1]
+	}
+	return validReturns
+}