Solution.go

func numberCount(a int, b int) int {
	num := strconv.Itoa(b)
	f := make([][1 << 10]int, len(num))
	for i := range f {
		for j := range f[i] {
			f[i][j] = -1
		}
	}
	var dfs func(pos, mask int, limit bool) int
	dfs = func(pos, mask int, limit bool) int {
		if pos >= len(num) {
			if mask != 0 {
				return 1
			}
			return 0
		}
		if !limit && f[pos][mask] != -1 {
			return f[pos][mask]
		}
		up := 9
		if limit {
			up = int(num[pos] - '0')
		}
		ans := 0
		for i := 0; i <= up; i++ {
			if mask>>i&1 == 1 {
				continue
			}
			nxt := mask | 1<<i
			if mask == 0 && i == 0 {
				nxt = 0
			}
			ans += dfs(pos+1, nxt, limit && i == up)
		}
		if !limit {
			f[pos][mask] = ans
		}
		return ans
	}
	y := dfs(0, 0, true)
	num = strconv.Itoa(a - 1)
	for i := range f {
		for j := range f[i] {
			f[i][j] = -1
		}
	}
	x := dfs(0, 0, true)
	return y - x
}