Alternating Digit Sum - Practice Coding Problems

Alternating Digit Sum - Problem

Math Easy

You are given a positive integer n. Each digit of n has a sign according to the following rules:

The most significant digit is assigned a positive sign.
Each other digit has an opposite sign to its adjacent digits.

Return the sum of all digits with their corresponding sign.

Input & Output

Example 1 — Three Digit Number

$ Input: n = 521

› Output: 4

💡 Note: Digits are 5, 2, 1. Signs are +, -, +. So we get +5 + (-2) + (+1) = 5 - 2 + 1 = 4

Example 2 — Four Digit Number

$ Input: n = 111

› Output: 1

💡 Note: Digits are 1, 1, 1. Signs are +, -, +. So we get +1 + (-1) + (+1) = 1 - 1 + 1 = 1

Example 3 — Single Digit

$ Input: n = 886

› Output: 22

💡 Note: Digits are 8, 8, 6. Signs are +, -, +. So we get +8 + (-8) + (+6) = 8 - 8 + 6 = 6

Constraints

1 ≤ n ≤ 10⁹

Visualization

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Understanding the Visualization

Input Number

Given positive integer n = 521

Assign Signs

Most significant digit positive, then alternate

Calculate Sum

Add all digits with their assigned signs

Key Takeaway

🎯 Key Insight: Start with positive sign for the leftmost digit and alternate signs for each subsequent digit

Asked in

LC LeetCode 15 M Microsoft 8

The key insight is to process each digit with alternating signs starting positive for the most significant digit. Both string conversion and mathematical digit extraction work well. Best approach is string conversion for simplicity. Time: O(d), Space: O(d) for string or O(1) for math approach.

Common Approaches

Approach	Time	Space	Notes
✓ String Conversion Approach	O(d)	O(d)	Convert number to string and process each digit with alternating signs
Mathematical Digit Extraction	O(d)	O(1)	Extract digits mathematically using modulo and division operations

String Conversion Approach — Algorithm Steps

Convert integer n to string
Loop through each character from left to right
Convert character to digit and apply alternating sign
Add to running sum

Visualization

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Step-by-Step Walkthrough

Convert to String

Convert integer to string for easy digit access

Apply Signs

First digit positive, then alternate minus/plus

Sum Result

Add all signed digits together

Code -

solution.c — C

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

int solution(int n) {
    char s[20];
    sprintf(s, "%d", n);
    int result = 0;
    int positive = 1;
    
    for (int i = 0; i < strlen(s); i++) {
        int digit = s[i] - '0';
        if (positive) {
            result += digit;
        } else {
            result -= digit;
        }
        positive = !positive;
    }
    
    return result;
}

int main() {
    int n;
    scanf("%d", &n);
    int result = solution(n);
    printf("%d\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(d)

Where d is the number of digits, we iterate through each digit once

✓ Linear Growth

Space Complexity

O(d)

String conversion requires space proportional to number of digits

✓ Linear Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

String Conversion Approach — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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