consider the input size of an algorithm is 'n'. As the space taken by the algorithm is 'n' the time taken wil be vary based on the algorithm such as O(n),O(n^2),O(n^3) etc.But the input size is always n.
Therefore consider S(n) as space complexity i.e is s(n) =n and the time complexity as T(n)=n or n^2 or n^3 etc.
so S(n)<=C.T(n)
Therefore S(n)=O(T(n)).