\(\text{Code}\)

  for(int S = 0; S < (1 << n); S++)
    for(int T = S; T; T = (T - 1) & S)
      //do something

\(\text{Explanation}\)

\(T\) 为 \(S\) 的子集，且枚举子集的复杂度是 \(O(3^n)\)