Codingame puzzle walkthough: Number of Digits

My implementation for the codingame puzzle: Number of Digits in golang

September 2024

Find the puzzle on Codingame: Puzzle link.

Prompt

Count the number of times the digit k appears in all non-negative integers less than or equal to a given integer n.

Parsing

For the setup of this puzzle, I’m storing each digit of the max number in an array. This is splitting the “unit”, the “ten”, the “hundred”, etc… numbers. This is what will allow us to save time by saving the results of some exploration.

I’m also creating a memo array that will be used to save those results.

    digits := make([]int, 9)
    memo = make([][]int, 9)
    for i := range memo {
        row := make([]int, 9)
        for j := range row {
            row[j] = -1
        }
        memo[i] = row
    }

    length := 0
    for (n > 0) {
        digits[length] = n % 10
        length++
        n = int(math.Floor(float64(n / 10)))
    }

The algo

Now the hard part! Here’s the code without the memoization to start.

Starting at the digit representing the biggest unit (ie: the 3 in 341 for the hundreds) I’m iterating from 0 to that digit, adding one to our count if it’s our target, then launching the same algo on the next biggest digit (ie: the 4 in 341 for the tens).

The limit is used to make sure I’m not counting digit above the number. the limit stays on only when I launch the search from the last possible digit. If the search is from an earlier digit, I remove the limit iterate from 0 to 9.

It’s a kind of Depth-First-Search algorithm.

// fmt.Println(dfs(digits, k, length-1, 0, true))
func dfs(digits []int, target, pos, count int, limit bool) int {
    if pos < 0 {
        return count
    }

    ans := 0
    bound := 9
    if limit {
        bound = digits[pos]
    }

    for i := 0; i<=bound;i++{
        add := 0
        if i == target {
            add = 1
        }
        ans += dfs(digits, target, pos -1, count + add, limit && i == bound)
    }

    return ans
}

This means that for 341, the search will:

And remember that each time the target is found, I’m incrementing our count by one

I can now add the memo. Before iterating through the numbers, if limit is false (most of the time) I’m checking if this combinaison of position and count were already checked. If they were, I just return the count found last time.

If they weren’t, I do the calculation, then save the result!

// fmt.Println(dfs(digits, k, length-1, 0, true))
func dfs(digits []int, target, pos, count int, limit bool) int {
    if pos < 0 {
        return count
    }

    if !limit && memo[pos][count] != -1 {
        return memo[pos][count]
    }

    ans := 0
    bound := 9
    if limit {
        bound = digits[pos]
    }

    for i := 0; i<=bound;i++{
        add := 0
        if i == target {
            add = 1
        }
        ans += dfs(digits, target, pos -1, count + add, limit && i == bound)
    }

    if !limit {
        memo[pos][count] = ans
    }

    return ans
}

Complete solution

If you have any questions or suggestions, send me a message at me@krayorn.com or a DM on Twitter/X

package main

import "fmt"
import "math"

var memo [][]int

func main() {
    var n int
    fmt.Scan(&n)
    
    var k int
    fmt.Scan(&k)
    
    digits := make([]int, 9)
    memo = make([][]int, 9)
    for i := range memo {
        row := make([]int, 9)
        for j := range row {
            row[j] = -1
        }
        memo[i] = row
    }

    length := 0
    for (n > 0) {
        digits[length] = n % 10
        length++
        n = int(math.Floor(float64(n / 10)))
    }
    fmt.Println(dfs(digits, k, length-1, 0, true))
}

func dfs(digits []int, target, pos, count int, limit bool) int {
    if pos < 0 {
        return count
    }

    if !limit && memo[pos][count] != -1 {
        return memo[pos][count]
    }

    ans := 0
    bound := 9
    if limit {
        bound = digits[pos]
    }

    for i := 0; i<=bound;i++{
        add := 0
        if i == target {
            add = 1
        }
        ans += dfs(digits, target, pos -1, count + add, limit && i == bound)
    }

    if !limit {
        memo[pos][count] = ans
    }

    return ans
}

go