Nested for loop in Big Oh Complexity

Question

for(int i = 0; i < n; i++) {
    for(int j = 0; j < i; j++){
        //do swap stuff, constant time
    }
}

I read that single for loop is O(N) a

Answer 1

A function that loops from i = 1 to n and then has a inner loop that goes from 1 to i would go though a number of iteration equal to this formula:

n(n+1)/2

As you can see, when we get rid of everything besides the main exponent, you end with O(n^2)