for(int i=0 ; i<strlen(s) ; i++)

strlen(s) has $$$O(n)$$$ complexity so this whole loop has $$$O(n^2)$$$ complexity.

strlen() is O(n)

the way strlen works is that it will scan and count, until it finds a null character: every time you call strlen, it scans again and again from the beginning, so it's inefficient, while s.size() is constant