leetcode518+硬币组合种类数，Dp

https://leetcode.com/problems/coin-change-2/description/

class Solution {
public:
    int change(int amount, vector& coins) {
        vector dp(amount+1); //dp[x] 总金额为x时硬币组合数
        dp[0] = 1; //面值为0只有1种组合方式
        for(int i=0; i=coins[i]){
                    dp[j] += dp[j-coins[i]];
                }
            }
        }
        return dp[amount];
    }
};