Running Sum of 1d Array

Problem Description

Given an array nums. We define a running sum of an array as runningSum[i] = sum(nums[0]…nums[i]). Return the running sum of nums.

Examples

Example 1:

Input: nums = [1,2,3,4]
Output: [1,3,6,10]
Explanation: Running sum is obtained as follows: [1, 1+2, 1+2+3, 1+2+3+4].

Example 2:

Input: nums = [1,1,1,1,1]
Output: [1,2,3,4,5]
Explanation: Running sum is obtained as follows: [1, 1+1, 1+1+1, 1+1+1+1, 1+1+1+1+1].

Complexity Analysis

*** Time Complexity:** $O(n)$

*** Space Complexity:** $O(n)$

Constraints

• 1<= nums.length <= 1000
• 10^6 <= nums[i] <= 10^6

Solution

Approach

The approach involves creating a result array of the same length as the input array, initialized to 1. We maintain a cumulative sum variable (sum1) that starts at 0. As we iterate through the input array, we update each element of the result array to be the current element plus the cumulative sum, then update the cumulative sum by adding the current element. This process ensures that each element in the result array represents the running sum of the input array up to that point.

Code in Different Languages

C++

Written by @ImmidiSivani

class Solution {
public:
    std::vector<int> runningSum(std::vector<int>& nums) {
        std::vector<int> res(nums.size(), 1);
        int sum1 = 0;
        for (int i = 0; i < nums.size(); ++i) {
            res[i] = nums[i] + sum1;
            sum1 += nums[i];
        }
        return res;
    }
};

Java

Written by @ImmidiSivani

class Solution {
    public int[] runningSum(int[] nums) {
        int[] res = new int[nums.length];
        int sum1 = 0;
        for (int i = 0; i < nums.length; i++) {
            res[i] = nums[i] + sum1;
            sum1 += nums[i];
        }
        return res;
    }
}

Python

Written by @ImmidiSivani

class Solution:
    def runningSum(self, nums: List[int]) -> List[int]:
        res=[1]*(len(nums))
        sum1=0
        for i in range(len(nums)):
            res[i]=nums[i]+sum1
            sum1+=nums[i]
        return res

References

• LeetCode Problem: Running Sum of 1d Array

• Solution Link: Running Sum of 1d Array