Date: 2026-06-06
Time: 15:14
alternating-digit-sum/solution.pyThis file solves LeetCode 2544: Alternating Digit Sum. It computes the alternating sum of a positive integer's digits, where the most significant digit gets a positive sign, the next gets negative, and so on. For example, 521 → +5 - 2 + 1 = 4.
sumofdigits(n: int) -> int — The sole function. Takes a positive integer n (1 ≤ n ≤ 10⁹) and returns the alternating digit sum.
The contract is simple: the first (leftmost) digit is always added, the second subtracted, third added, etc.
1. Convert n to its string representation s, which naturally gives digits in most-significant-first order.
2. Use a generator expression that iterates over enumerate(s), producing (index, digit_char) pairs.
3. For each pair, compute int(d) * (-1) ** i — when i is even the sign is +1, when odd it's -1.
4. sum() accumulates the result.
The (-1) ** i trick is the core idiom: it maps even indices to +1 and odd indices to -1 without branching.
str to walk digits left-to-right avoids the common pitfall of extracting digits via % 10 (which yields them in reverse order and would require tracking the total digit count to assign signs correctly).(-1) ** i is idiomatic Python for alternating-sign sequences. It's concise but performs an exponentiation per digit — for a max 10-digit number this is negligible.Imports: None — pure standard Python.
Imported by: The test_solution.py in the same directory, plus ~350+ test files across other problem directories. The "Imported By" list in the prompt is misleading — those other test files don't actually depend on *this* solution's logic. That list reflects how the test harness resolves imports across the monorepo (likely a shared test runner pattern), not true logical coupling.
n must be a positive integer. The function doesn't validate this — passing 0 yields 0 (technically correct since str(0) is "0"), but negative numbers would include a - character and int("-") would raise ValueError.(-1)**0 = 1).None. Invalid input (negative numbers, non-integers) will raise exceptions from str()/int() — this is appropriate for a LeetCode solution where input constraints are guaranteed.
alternating-digit-sum/test_solution.py — See which edge cases are covered (single digit, large numbers, etc.)alternating-digit-sum/review.md — Check if the review flagged the (-1)**i approach vs. a conditional or multiplier toggledifference-between-element-sum-and-digit-sum-of-an-array/solution.py:sumofdigits — Compare digit-extraction approach in a related digit-sum problemdigit-extraction-patterns — Compare str(n) iteration vs. divmod loop vs. math.log10 across the repo's digit-manipulation problemsalternating-sign-via-exponentiation — (-1) ** i produces +1 for even i and -1 for odd i, giving the most-significant digit a positive signstr-conversion-for-left-to-right-digits — Converting to string yields digits in most-significant-first order, which is essential since the sign assignment depends on position from the leftno-input-validation — The function assumes n ≥ 1 per LeetCode constraints; passing negative integers would raise ValueError on the - charactero-d-time-and-space — Where d is the number of digits (at most 10); the string allocation and generator are both O(d)