Cumulative sum of Subarray

A cumulative sum Array is an array of all the sums of an array till that index.

For example, 

Arr={1,2,3,4}

for i=0 cumulative sum will be 1

for i=1 cumulative sum will be 1+2=3

for i=2 cumulative sum will be 1+2+3=6

for i=3 cumulative sum will be 1+2+3+4=10

So the cumulative array becomes {1,3,6,10}

One way of calculating it is by using two loops nested and calculating um and storing it in an array. As

int k=0;                           

while(k<n){
  int sum=0;
  for(int j=0;j<k+1;j++){
    sum=sum+arr[j];
    
  }
  ans[k]=sum;
  k++;
}

The other and the most efficient way of calculating the cumulative array is by adding the current element to the previous sum.

Arr={1,2,3,4}

cumArr[] is cumulative array.

1. cumArr[0]=1
2. cumArr[1]=prev sum+Arr[1] = 1 + 2 = 3
3. cumArr[2]=prev sum+Arr[2] = 3 + 3 = 6
4. cumArr[3]=prev sum+Arr[1] = 6 + 4= 10

It can be executed as

int sum=0;
for(int i=0;i<n;i++){
  sum=sum+arr[i];
  ans[i]=sum;
}

Brainteaser

What do you think will be the time complexity of both of these approaches ??