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float(“INF”)表示为正无穷；
float("-INF")表示负无穷

def coinChange(coins: List[int], amount: int):
    def dp(n):
        # base case
        if n == 0: return 0
        if n < 0: return -1
        # 求最小值，所以初始化为正无穷
        res = float('INF')
        for coin in coins:
            subproblem = dp(n - coin)
            # 子问题无解，跳过
            if subproblem == -1: continue
            res = min(res, 1 + subproblem)

        return res if res != float('INF') else -1

    return dp(amount)