def search_n(s, c, n): size = 0 for i, x in enumerate(s): if x == c: size += 1 if size == n: return i return -1 print(search_n("fdasadfadf", "a", 3))# 结果为7,正确 print(search_n("fdasadfadf", "a", 30))# 结果为-1,正确