< Back 1480. Running Sum of 1d Array Using the original array, we just traverse the array from array[1] to array[n], adding to array[i] the value of the previous element in the array, array[i-1]. The solution is as follows: class Solution: def runningSum(self, nums: List[int]) -> List[int]: for i in range(1, len(nums)): nums[i] += nums[i - 1] return nums _ Time Complexity: O(n) - We traverse the array once. _ Space Complexity: O(1) - We modify the array in-place.