go: add «38. Count and Say»

URL:	https://leetcode.com/problems/count-and-say/
Signed-off-by: Matej Focko <me@mfocko.xyz>

go/count-and-say.go

@@ -0,0 +1,31 @@
+package main
+
+import "fmt"
+
+func countAndSay(n int) string {
+	var build func(int, string) string
+	build = func(idx int, result string) string {
+		if idx >= n {
+			return result
+		}
+
+		previous, left, count := result, 0, 0
+		result = ""
+
+		for right, c := range previous {
+			if rune(previous[left]) == c {
+				count++
+				continue
+			}
+
+			result += fmt.Sprint(count) + string(previous[left])
+			left = right
+			count = 1
+		}
+
+		result += fmt.Sprint(count) + string(previous[left])
+		return build(idx+1, result)
+	}
+
+	return build(1, "1")
+}