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# How to solve Coding Challenge 1480. Running Sum of 1d Array

Nhut Nguyen
·Jun 1, 2022·

## Problem statement

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`.

### 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].
``````

### Example 3

``````Input: nums = [3,1,2,10,1]
Output: [3,4,6,16,17]
``````

### Constraints

• `1 <= nums.length <= 1000`.

• `-10^6 <= nums[i] <= 10^6`.

## Solution 1: Unchange `nums`

### Code

``````#include <vector>
#include <iostream>
using namespace std;
vector<int> runningSum(vector<int>& nums) {
vector<int> rs;
int s = 0;
for (int n : nums) {
s += n;
rs.push_back(s);
}
return rs;
}
void printResult(vector<int>& sums) {
cout << "[";
for (int s: sums) {
cout << s << ",";
}
cout << "]\n";
}
int main() {
vector<int> nums{1,2,3,4};
auto rs = runningSum(nums);
printResult(rs);
nums = {1,1,1,1,1};
rs = runningSum(nums);
printResult(rs);
nums = {3,1,2,10,1};
rs = runningSum(nums);
printResult(rs);
}
``````
``````Output:
[1,3,6,10,]
[1,2,3,4,5,]
[3,4,6,16,17,]
``````

### Complexity

• Runtime: `O(N)`, where `N = nums.length`.

• Extra space: `O(1)`.

## Solution 2: Change `nums`

If `nums` is allowed to be changed, you could use it to store the result directly.

### Code

``````#include <vector>
#include <iostream>
using namespace std;
vector<int> runningSum(vector<int>& nums) {
for (int i = 1; i < nums.size(); i++) {
nums[i] += nums[i - 1];
}
return nums;
}
void printResult(vector<int>& sums) {
cout << "[";
for (int s: sums) {
cout << s << ",";
}
cout << "]\n";
}
int main() {
vector<int> nums{1,2,3,4};
auto rs = runningSum(nums);
printResult(rs);
nums = {1,1,1,1,1};
rs = runningSum(nums);
printResult(rs);
nums = {3,1,2,10,1};
rs = runningSum(nums);
printResult(rs);
}
``````
``````Output:
[1,3,6,10,]
[1,2,3,4,5,]
[3,4,6,16,17,]
``````

### Complexity

• Runtime: `O(N)`, where `N = nums.length`.

• Extra space: `O(1)`.