刷题网散尔尔
followup1: 返回所有可能组合
followup2: 假如每个硬币个数有限，比如：
coins = [3, 2, 5],
counts = [3,10,1] - (4 coins 1, 10 coins of 3 etc)
amount = 11
刷题网散尔尔
followup1: 返回所有可能组合
followup2: 假如每个硬币个数有限，比如：
coins = [3, 2, 5],
counts = [3,10,1] - (4 coins 1, 10 coins of 3 etc)
amount = 11