第四题：dp，递归深度太大。 if __name__ == '__main__': k = input() k = int(k) S = 'S' A = 'A' B = 'B' C = 'C' d = [0] def Foot(s, k): if k == 0: if s == S: d[0] += 1 return 0 k -= 1 if s == S: Foot(A, k) Foot(B, k) Foot(C, k) return if s == A: Foot(S, k) Foot(B, k) Foot(C, k) return if s == B: Foot(S, k) Foot(A, k) Foot(C, k) return if s == C: Foot(S, k) Foot(A, k) Foot(B, k) return Foot(S, k)