Date: 2026-06-06
Time: 17:47
This file solves LeetCode 2570: Merge Two 2D Arrays by Summing Values. It owns a single responsibility: merging two sorted arrays of [id, value] pairs into one sorted array, summing values when IDs match. It's the canonical merge step from merge sort, adapted for key-value pairs.
merge_nums(nums1, nums2) -> list[list[int]]The only function. Contract:
[id, value] pairs, each sorted by id with unique IDs within each list.id. Where both inputs share an ID, the output contains one entry with summed values. Where an ID appears in only one input, it passes through unchanged.Two-pointer merge. This is the textbook sorted-merge pattern: maintain one pointer per input, advance whichever points at the smaller key, and drain the remainder when one input is exhausted. The three-phase structure (main merge loop, drain nums1, drain nums2) is identical to the merge step in merge sort.
No hash maps, no defaultdict, no Counter — the solution exploits the sorted precondition to achieve O(n + m) time with O(1) auxiliary space (beyond the output list). This is the optimal approach when inputs are pre-sorted.
Imports: None. Pure Python, no standard library usage.
Imported by: Only its own testsolution.py. The "Imported By" list in the prompt is misleading — those are test files for *other* problems that happen to share a test harness or runner, not actual importers of mergenums.
1. Initialize result = [] and two pointers i, j = 0, 0.
2. Main loop (lines 16–24): while both pointers are in bounds, compare nums1[i][0] vs nums2[j][0]:
nums1[i] as-is, advance i.nums2[j] as-is, advance j.[id, sumofvalues], advance both.3. Drain loops (lines 25–30): append any remaining elements from whichever input wasn't fully consumed.
4. Return the merged result.
Data flows linearly — each element from both inputs is visited exactly once, and the output is built by appending in sorted order.
id. If violated, the output won't be sorted and merges will be missed.id (given sorted inputs), because the two-pointer merge always picks the smallest unconsumed key.[id, sum] lists for matches. It doesn't modify nums1 or nums2 in place (though it does alias non-matched entries rather than copying them).None. The function trusts its inputs entirely — no bounds checking, no type validation, no handling of empty lists (which work correctly by falling through to the drain loops). This is appropriate for a LeetCode solution where inputs are guaranteed well-formed.
merge-two-2d-arrays-by-summing-values/test_solution.py — See what edge cases are covered (empty inputs, no overlap, full overlap)merge-similar-items/solution.py — Nearly identical problem (merge by key, sum values) — compare whether it uses the same two-pointer approach or a hash mapmerge-two-sorted-lists/solution.py — Same merge pattern applied to linked lists instead of arraysinput-aliasing — The function appends references to input sublists rather than copies; worth checking if any caller mutates the resultsquares-of-a-sorted-array/solution.py:sortedSquares — Another two-pointer technique on sorted input, worth comparing the pointer movement logicmerge-nums-linear-time — merge_nums visits each element exactly once, achieving O(n + m) time complexitymerge-nums-no-input-mutation — The function never modifies nums1 or nums2; matched entries produce new lists, unmatched entries are aliasedmerge-nums-sorted-precondition — Correctness depends on both inputs being sorted by ID; unsorted inputs produce incorrect results silentlymerge-nums-aliases-unmatched — For IDs appearing in only one input, the output list holds a reference to the original [id, value] sublist, not a copy