Alternating Digit Sum - Complete Solution Guide

Alternating Digit Sum is LeetCode problem 2544, a Easy level challenge. This complete guide provides step-by-step explanations, multiple solution approaches, and optimized code in python3.

Problem Statement

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 . Example 1: Input: n = 521 Output: 4 Explanation: (+5) + (-2) + (+1) = 4. Example 2: Input: n = 111 Output: 1 Explanation: (+1) + (-1) + (+1) = 1. Example 3: Input: n = 886996 Output: 0 Explanation: (+8) + (-8) + (+6) + (-9)

Detailed Explanation

The problem asks you to calculate the alternating digit sum of a positive integer. The most significant digit (leftmost) is added to the sum. The next digit is subtracted. The following digit is added, and so on, alternating between addition and subtraction. The function should return the final sum.

Solution Approach

The Python solution converts the input integer `n` into a string `s`. It initializes a variable `ans` to 0 (the accumulated sum) and `sign` to 1 (initial positive sign). The code iterates through each digit in the string `s`. In each iteration, it converts the digit back to an integer, multiplies it by the current `sign`, and adds the result to `ans`. Then, it flips the `sign` by multiplying it by -1. Finally, it returns the calculated `ans`.

Step-by-Step Algorithm

1. Step 1: Convert the input integer `n` to a string `s`.
2. Step 2: Initialize `ans` to 0 and `sign` to 1.
3. Step 3: Iterate through each character (digit) in the string `s`.
4. Step 4: Convert the character to an integer, multiply it by `sign`, and add the result to `ans`.
5. Step 5: Change the sign by multiplying `sign` by -1.
6. Step 6: Repeat steps 3-5 until all digits are processed.
7. Step 7: Return `ans`.

Key Insights

• Insight 1: The problem can be efficiently solved by iterating through the digits of the number as strings.
• Insight 2: Using a variable to track the sign (+1 or -1) simplifies the alternating addition/subtraction.
• Insight 3: Converting the integer to a string allows easy access to individual digits.

Complexity Analysis

Time Complexity: O(log(n))

Space Complexity: O(log(n))

Topics

This problem involves: Math.

Companies

Asked at: eBay.