Problem:
Score : 500 points

Problem Statement

Find the number of integers between 1 and N (inclusive) that contains exactly K non-zero digits when written in base ten.

Constraints

1 \leq N < 10^{100}

1 \leq K \leq 3

Input

Input is given from Standard Input in the following format:

N
K

Output

Print the count.

Sample Input 1

100
1

Sample Output 1

19

The following 19 integers satisfy the condition:

1,2,3,4,5,6,7,8,9,10,20,30,40,50,60,70,80,90,100

Sample Input 2

25
2

Sample Output 2

14

The following 14 integers satisfy the condition:

11,12,13,14,15,16,17,18,19,21,22,23,24,25

Sample Input 3

314159
2

Sample Output 3

937

Sample Input 4

9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999
3

Sample Output 4

117879300

Solution in Go (1.6) (judge verdict: Accepted)

package main

import (
	"bufio"
	_ "container/heap"
	"fmt"
	"os"
	_ "sort"
	"strconv"
)

// I/O
type Scanner struct {
	sc *bufio.Scanner
}

func NewScanner() *Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &Scanner{sc}
}

func (s *Scanner) nextStr() string {
	s.sc.Scan()
	return s.sc.Text()
}

func (s *Scanner) nextInt() int {
	i, e := strconv.Atoi(s.nextStr())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *Scanner) nextFloat() float64 {
	f, e := strconv.ParseFloat(s.nextStr(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *Scanner) nextRuneSlice() []rune {
	return []rune(s.nextStr())
}

func (s *Scanner) nextIntSlice(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextInt()
	}
	return res
}

func (s *Scanner) nextFloatSlice(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextFloat()
	}
	return res
}

// Arithmetic
func max(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m < i {
			m = i
		}
	}
	return m
}

func min(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m > i {
			m = i
		}
	}
	return m
}

func abs(x int) int {
	if x > 0 {
		return x
	}
	return -x
}

func pow(x, y int) int {
	res := 1
	for i := 0; i < y; i++ {
		res *= x
	}
	return res
}

func ceil(a, b int) int {
	if a%b == 0 {
		return a / b
	} else {
		return a/b + 1
	}
}

func gcd(a, b int) int {
	if b == 0 {
		return a
	}
	return gcd(b, a%b)
}

func lcm(a, b int) int {
	return a / gcd(a, b) * b
}

// Sort
type RuneSlice []rune

func (a RuneSlice) Len() int           { return len(a) }
func (a RuneSlice) Less(i, j int) bool { return a[i] < a[j] }
func (a RuneSlice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

// Main
const MOD = int(1e+9) + 7
const INF = 1 << 60

func main() {
	sc := NewScanner()
	wtr := bufio.NewWriter(os.Stdout)
	defer wtr.Flush()
	N, K := sc.nextStr(), sc.nextInt()
	keta := len(N)
	dp0 := make([][]int, keta+1)
	dp1 := make([][]int, keta+1)
	for i := 0; i < keta+1; i++ {
		dp0[i] = make([]int, K+1)
		dp1[i] = make([]int, K+1)
	}
	dp1[0][0] = 1
	for i := 0; i < keta; i++ {
		n, _ := strconv.Atoi(string(N[i]))
		for j := 0; j <= K; j++ {
			for x := 0; x < 10; x++ {
				if x == n {
					if x == 0 {
						dp1[i+1][j] += dp1[i][j]
						dp0[i+1][j] += dp0[i][j]
					} else if 0 < j {
						dp1[i+1][j] += dp1[i][j-1]
						dp0[i+1][j] += dp0[i][j-1]
					}
				} else {
					if x == 0 {
						dp0[i+1][j] += dp0[i][j]
					} else if 0 < j {
						dp0[i+1][j] += dp0[i][j-1]
					}
					if x < n {
						if x == 0 {
							dp0[i+1][j] += dp1[i][j]
						} else if 0 < j {
							dp0[i+1][j] += dp1[i][j-1]
						}
					}
				}
			}
		}
	}

	fmt.Fprintln(wtr, dp0[keta][K]+dp1[keta][K])
}