Date: 2026-06-06
Time: 19:22
sum-of-digits-of-string-after-convert/solution.pyThis file implements LeetCode problem 1945: *Sum of Digits of String After Convert*. It owns the complete solution and its test suite in a single module — consistent with the repo's convention of co-locating solution and tests per problem directory.
The problem: given a lowercase string s, replace each letter with its 1-indexed alphabet position (a=1, ..., z=26), concatenate those numbers into a single numeric string, then repeatedly sum that string's digits k times. Return the final integer.
Solution.getLucky(self, s: str, k: int) -> int — The core algorithm. Contract:
s contains only lowercase English letters.k >= 1 (at least one digit-sum transform is always applied).TestGetLucky — Seven unit tests covering LeetCode's three examples, single-character edge cases ('a' and 'z'), a repeated-character case ('zzzz'), and a convergence test with high k.
The algorithm has two phases:
1. Convert (line 14): Each character maps to its alphabet position via ord(c) - ord('a') + 1, producing strings like "26" for 'z'. These are concatenated — not summed — so "zbax" becomes "2621124", not [26, 2, 1, 24].
2. Transform (lines 15–17): The first digit-sum happens immediately on numstr (line 15). The remaining k - 1 transforms loop over the digits of the running result integer. This split avoids converting back to string unnecessarily on the first pass — numstr is already a string, so summing its digits directly is natural.
After a single digit-sum, the result is at most 9 * len(numstr). For the maximum input (1000 characters of 'z'), numstr has 2000 digits, so the first sum is at most 18,000 — a 5-digit number. The second sum is at most 45. By the third iteration the value is a single digit and stays fixed, which is why testhighk with k=10 works correctly.
if _name == "main_" guard lets each file run standalone via python solution.py.sum(int(d) for d in str(result)) is the idiomatic Python pattern for digit summation — no modular arithmetic needed.int → str → sum.Imports: Only unittest from stdlib — no external dependencies.
Imported by: The testsolution.py files listed in the context don't actually import this file — they're test files for *other* problems that happen to share the same structural pattern. The sum-of-digits-of-string-after-convert/testsolution.py is the one that tests this code (likely importing Solution from it).
k >= 1 is assumed — the code always performs at least one digit-sum (line 15), then loops k - 1 more times. Passing k=0 would still execute one sum, which would violate the problem spec if taken literally.a–z. The ord(c) - ord('a') + 1 mapping produces values 1–26; any other character would produce incorrect or negative values silently.result is a non-negative integer, so str(result) is always a valid digit string (no sign character to worry about).None. The code trusts its inputs per LeetCode convention — no validation, no exceptions. Invalid input (empty string, uppercase, non-alpha) produces silently wrong results rather than errors.
add-digits/solution.py — Same digit-sum-until-convergence pattern; likely uses the digital root formula 1 + (n-1) % 9 as an O(1) shortcutcalculate-digit-sum-of-a-string/solution.py — Related problem that groups digits before summing; compare the chunking approachdifference-between-element-sum-and-digit-sum-of-an-array/solution.py — Another digit-extraction problem; see if it uses the same int(d) for d in str(n) idiomdigital-root-convergence — Why repeated digit sums always converge to a single digit (the digital root), and when the closed-form 1 + (n-1) % 9 applies vs. when you must iteratesum-of-digits-of-string-after-convert/solution.py:getLucky — Trace through with s="leetcode", k=2 to see how the two-phase structure works end-to-endfirst-sum-on-string — getLucky computes the first digit-sum directly from the concatenated numeric string, avoiding an intermediate conversion to int (which could be thousands of digits long)k-minus-one-loop — The for loop runs k - 1 iterations, not k, because the initial sum on num_str counts as the first transformconvergence-by-k3 — For any valid input (up to 1000 lowercase letters), the result stabilizes to a single digit by the third transform at most, making large k values effectively no-ops after that pointno-input-validation — The function assumes s contains only a–z and k >= 1; violating either produces silently wrong results, not errorsord-offset-mapping — Letter-to-position mapping uses ord(c) - ord('a') + 1, producing multi-character strings for letters j–z (positions 10–26), which is why the concatenation step is necessary rather than simple per-character digit extraction