LeetCode 2570: Merge Two 2D Arrays by Summing Values ·

Table of Contents

Meta Data
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Difficulty: easy First Attempt: 2026-01-31

Submissions: Accepted (first try)

Approach
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Both arrays are sorted by id with unique ids in each. It is merging two sorted sequences (like LC 88), except matching ids add their values:

Same id: output [id, sum of both values] and advance both pointers.
Different: output the smaller id and advance that pointer.
Once one side runs out, append the rest of the other.

My Solution
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class Solution:
    def mergeArrays(self, nums1: List[List[int]], nums2: List[List[int]]) -> List[List[int]]:
        i = j = 0
        res = []
        while i < len(nums1) and j < len(nums2):
            if nums1[i][0] == nums2[j][0]:
                res.append([nums1[i][0], nums1[i][1] + nums2[j][1]])
                i += 1
                j += 1
            elif nums1[i][0] < nums2[j][0]:
                res.append(nums1[i])
                i += 1
            else:
                res.append(nums2[j])
                j += 1
        return res + nums1[i:] + nums2[j:]

My original handled “one side is exhausted” inside the same loop (while i < n or j < m with three cases inside). Looping only while both sides have elements and appending the rest afterwards is shorter.

Complexity
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Time O(n + m), space O(n + m).

Mistakes to Avoid
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Collecting into a dict and sorting: it works, but ignores that the inputs are sorted and adds O(k log k).
Forgetting to append the leftovers after the loop.

Interview Explanation
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Both arrays are sorted by id, so I merge with two pointers: equal ids add their values and advance both; otherwise I output the smaller id and advance it. When one side runs out, I append the rest of the other. Time O(n + m).