add new solutions

solutions/2/q216/solution.go

@@ -0,0 +1,37 @@
+package q216
+
+// k numbers sum up to n
+
+func calc(k int, buf []int, offset int, remSum int, ret [][]int) [][]int {
+	if offset == k-1 && remSum > 0 && remSum < 10 {
+		buf[offset] = remSum
+		cp := make([]int, len(buf))
+		copy(cp, buf)
+		return append(ret, cp)
+	}
+
+	remK := k - offset
+	minI := 1
+	if offset > 0 {
+		minI = buf[offset-1] + 1
+	}
+	maxI := 9 - remK + 1
+
+	for i := minI; i <= maxI; i++ {
+		minSum := (i + i + remK - 1) * remK / 2
+		maxSum := (19 - remK) * remK / 2
+		if remSum < minSum || remSum > maxSum {
+			continue
+		}
+
+		buf[offset] = i
+		ret = calc(k, buf, offset+1, remSum-i, ret)
+	}
+	return ret
+}
+
+func combinationSum3(k int, n int) [][]int {
+	return calc(k, make([]int, k), 0, n, [][]int{})
+}
+
+var _ = combinationSum3