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

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)
}