go: add «523. Continuous Subarray Sum»

Signed-off-by: Matej Focko <me@mfocko.xyz>

go/continuous-subarray-sum.go

@@ -0,0 +1,22 @@
+package continuous_subarray_sum
+
+func checkSubarraySum(nums []int, k int) bool {
+	first_index_of := make(map[int]int)
+	first_index_of[0] = -1
+
+	remainder := 0
+	for i, x := range nums {
+		remainder = (remainder + x) % k
+
+		j, seen := first_index_of[remainder]
+		if seen {
+			if i-j > 1 {
+				return true
+			}
+		} else {
+			first_index_of[remainder] = i
+		}
+	}
+
+	return false
+}