Count Digits Frequency - Problem

Given a positive integer, count how many times each digit (0-9) appears in the number.

Task: Return an array of size 10 where the index represents the digit (0-9) and the value at that index represents the frequency of that digit in the given number.

Example: For the number 1223, digit 1 appears once, digit 2 appears twice, digit 3 appears once, and all other digits appear zero times.

Input & Output

Example 1 — Basic Case

$ Input: num = 1223

› Output: [0,1,2,1,0,0,0,0,0,0]

💡 Note: Digit 0 appears 0 times, digit 1 appears 1 time, digit 2 appears 2 times, digit 3 appears 1 time, digits 4-9 appear 0 times each

Example 2 — Single Digit

$ Input: num = 7

› Output: [0,0,0,0,0,0,0,1,0,0]

💡 Note: Only digit 7 appears once, all other digits appear 0 times

Example 3 — With Zeros

$ Input: num = 1002

› Output: [2,1,1,0,0,0,0,0,0,0]

💡 Note: Digit 0 appears 2 times, digit 1 appears 1 time, digit 2 appears 1 time

Constraints

0 ≤ num ≤ 10⁹
num is a non-negative integer

Visualization

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Asked in

a Amazon 15 G Google 12 M Microsoft 8

The key insight is to maintain a frequency counter for each digit (0-9) and process each digit in the number exactly once. The mathematical approach is most efficient, using modulo operations to extract digits without string conversion. Time: O(d), Space: O(1) where d is the number of digits.

Common Approaches

✓ Convert to String and Loop

⏱️ Time: O(d) Space: O(d)

Convert the integer to a string representation, then iterate through each character. For each digit character, increment the corresponding counter in our frequency array.

Mathematical Digit Extraction

⏱️ Time: O(d) Space: O(1)

Use mathematical operations to extract each digit from the number. Repeatedly take modulo 10 to get the last digit, then divide by 10 to remove it, continuing until the number becomes 0.

Convert to String and Loop — Algorithm Steps

Convert the number to a string
Create frequency array of size 10 initialized to 0
Loop through each character in string
Convert character back to digit and increment frequency array

Visualization

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Step-by-Step Walkthrough

Convert to String

Transform number 1223 into string "1223"

Initialize Array

Create frequency array [0,0,0,0,0,0,0,0,0,0]

Count Each Digit

Process each character and update frequency array

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void solution(int num, int* frequency) {
    // Convert number to string
    char numStr[20];
    sprintf(numStr, "%d", num);
    
    // Initialize frequency array for digits 0-9
    for (int i = 0; i < 10; i++) {
        frequency[i] = 0;
    }
    
    // Count frequency of each digit
    for (int i = 0; numStr[i] != '\0'; i++) {
        int digit = numStr[i] - '0';
        frequency[digit]++;
    }
}

int main() {
    int num;
    scanf("%d", &num);
    
    int frequency[10];
    solution(num, frequency);
    
    printf("[");
    for (int i = 0; i < 10; i++) {
        printf("%d", frequency[i]);
        if (i < 9) printf(",");
    }
    printf("]\n");
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(d)

Where d is the number of digits in the number - we visit each digit once

✓ Linear Growth

Space Complexity

O(d)

String conversion requires O(d) space to store the digit characters

✓ Linear Space

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Count Digits Frequency - Problem

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Convert to String and Loop — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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