go: add «2999. Count the Number of Powerful Integers»

URL:	https://leetcode.com/problems/count-the-number-of-powerful-integers/
Signed-off-by: Matej Focko <me@mfocko.xyz>

go/count-the-number-of-powerful-integers.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+func numberOfPowerfulInt(start int64, finish int64, limit int, s string) int64 {
+	calculate := func(x string) int64 {
+		if len(x) < len(s) {
+			return 0
+		}
+
+		if len(x) == len(s) {
+			if x >= s {
+				return 1
+			}
+			return 0
+		}
+
+		count := int64(0)
+
+		prefixLength := len(x) - len(s)
+		for i := 0; i < prefixLength; i++ {
+			if limit < int(x[i])-'0' {
+				count += int64(math.Pow(float64(limit+1), float64(prefixLength-i)))
+				return count
+			}
+
+			count += int64(int(x[i])-'0') * int64(math.Pow(float64(limit+1), float64(prefixLength-1-i)))
+		}
+
+		if x[len(x)-len(s):] >= s {
+			count++
+		}
+
+		return count
+
+	}
+
+	_start, _finish := fmt.Sprint(start-1), fmt.Sprint(finish)
+	return calculate(_finish) - calculate(_start)
+}