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题目：

题解：

class Solution {
public:int change(int amount, vector<int>& coins) {vector<int> dp(amount + 1), valid(amount + 1);dp[0] = 1;valid[0] = 1;for (int& coin : coins) {for (int i = coin; i <= amount; i++) {valid[i] |= valid[i - coin];}}if (!valid[amount])return 0;for (int& coin : coins) {for (int i = coin; i <= amount; i++) {dp[i] += dp[i - coin];}}return dp[amount];}
};