class Solution(object):
    def coinChange(self, coins, amount):
        """
        :type coins: List[int]
        :type amount: int
        :rtype: int
        """
        # INVALID 表示无效
        INVALID = amount+1
        dp = [INVALID]*(amount+1)
        
        # 初始化
        # 为啥初始化 dp[0] = 0
        # 表示使用coins组成0元的方法有几种? 
        dp[0] = 0

        for i in range(1, amount+1):
          
            for c in coins:
                # 如果使用coin 来组合数据
                if i>=c:
                  dp[i] = min(dp[i-c]+1,dp[i])


        if dp[amount] == INVALID:
          return -1
        return dp[amount]