C++ forward_list vs vector benchmarking - c++

Question

I was solving a leetcode question Add to Array-Form of Integer, the question is to add a vector with an integer, for example
Input: num = [2,1,5], k = 806
Output: [1,0,2,1]
Explanation: 215 + 806 = 1021
to solve this I have used two approaches, in place addition in the same vector
vector<int> _addToArrayForm(vector<int> A, int K) {
for (int i = A.size() - 1; i >= 0 && K > 0; --i) {
A[i] += K;
K = A[i] / 10;
A[i] %= 10;
}
while (K > 0) {
A.insert(A.begin(), K % 10);
K /= 10;
}
return A;
}
using forward_list
forward_list<int> addToArrayForm(vector<int>& num, int k) {
forward_list<int> res;
for(int i = num.size() -1; i >= 0; i--)
{
res.push_front((num[i] + k) % 10);
k = (num[i] + k) / 10;
}
while(k)
{
res.push_front(k % 10);
k /= 10;
}
return res;
}
I think using forward_list is better as the time complexity of push_front is O(1) while for A.insert(A.begin(), K % 10) time complexity is O(n) but on benchmarking both functions, the time taken by the vector approach is much lesser than forward_list on 10000 elements
forward_list took 0.000185 seconds
vector took 0.000019 seconds
I tried to calculate execution time, using this code
vector<int> num;
for(int i = 0; i < 10000; i++)
{
num.push_back(rand() % 10);
}
int k = 3432423;
clock_t start = clock();
forward_list<int> res = s.addToArrayForm(num, k);
clock_t end = clock();
printf("Took %f seconds\n", (((float)(end-start) / CLOCKS_PER_SEC)));
start = clock();
vector<int> res1 = s._addToArrayForm(num, k);
end = clock();
printf("Took %f seconds\n", (((float)(end-start) / CLOCKS_PER_SEC)));