LeetCode 1480: Running Sum of 1d Array

Meta Data

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Difficulty: easy First Attempt: 2022-11-23

• Submissions: Accepted (first try)

Approach

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The prefix sum itself (Prefix Sum): res[i] = res[i - 1] + nums[i]. In Python, itertools.accumulate does it directly.

My Solution

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class Solution:
    def runningSum(self, nums: List[int]) -> List[int]:
        for i in range(1, len(nums)):
            nums[i] += nums[i - 1]
        return nums

Complexity

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Time O(n), space O(1) in place.

Mistakes to Avoid

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• Start at index 1; element 0 stays as is.

Interview Explanation

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Adding the previous total to each element gives the prefix sums, done in place in O(n).