2015-day24 code pulled into algos and mathy packages

algos/combinations.go

@@ -0,0 +1,37 @@
+package algos
+
+// CombinationsInts returns all combinations of an input slice of a given length
+func CombinationsInts(nums []int, targetLength int) [][]int {
+	if targetLength > len(nums) {
+		panic("target length is greated than length of input slice")
+	}
+
+	var combos [][]int
+	// loop over starting points in the nums slice
+	for i := 0; i < len(nums); i++ {
+		combos = append(combos, helperCombinationsInts(nums[i:], targetLength, []int{})...)
+	}
+
+	return combos
+}
+
+func helperCombinationsInts(nums []int, length int, current []int) [][]int {
+	if len(current) == length {
+		return [][]int{append([]int{}, current...)}
+	}
+	var combos [][]int
+	for i := range nums {
+		// add value onto the current combo
+		current = append(current, nums[i])
+
+		// recurse with only the remaining numbers, then append any valid combos
+		// that were found
+		recurseResult := helperCombinationsInts(nums[i+1:], length, current)
+		combos = append(combos, recurseResult...)
+
+		// backtrack
+		current = current[:len(current)-1]
+	}
+
+	return combos
+}