Solution.go

func count(num1 string, num2 string, min_sum int, max_sum int) int {
	const mod = 1e9 + 7
	f := [23][220]int{}
	for i := range f {
		for j := range f[i] {
			f[i][j] = -1
		}
	}
	num := num2
	var dfs func(int, int, bool) int
	dfs = func(pos, s int, limit bool) int {
		if pos >= len(num) {
			if s >= min_sum && s <= max_sum {
				return 1
			}
			return 0
		}
		if !limit && f[pos][s] != -1 {
			return f[pos][s]
		}
		var ans int
		up := 9
		if limit {
			up = int(num[pos] - '0')
		}
		for i := 0; i <= up; i++ {
			ans = (ans + dfs(pos+1, s+i, limit && i == up)) % mod
		}
		if !limit {
			f[pos][s] = ans
		}
		return ans
	}
	a := dfs(0, 0, true)
	t := []byte(num1)
	for i := len(t) - 1; i >= 0; i-- {
		if t[i] != '0' {
			t[i]--
			break
		}
		t[i] = '9'
	}
	num = string(t)
	f = [23][220]int{}
	for i := range f {
		for j := range f[i] {
			f[i][j] = -1
		}
	}
	b := dfs(0, 0, true)
	return (a - b + mod) % mod
}