LeetCode 978. 最长湍流子数组

题目描述：

解法一（递归）：

class Solution {
public:
    int coinChange(vector<int>& coins, int amount) {
        if(amount==0) return 0;
        int res=INT_MAX;
        for(auto coin:coins){
            if(coin>amount)
                continue;
            int rt=coinChange(coins,amount-coin);
            if(rt==-1) continue;
            res=min(res,rt+1);
        }
        return res==INT_MAX? -1:res;
    }
};

解法二（带备忘录的递归）：

class Solution {
public:
    int coinChange(vector<int>& coins, int amount) {
        unordered_map<int,int> mp;
        return helper(coins,amount,mp);
    }
    int helper(vector<int>& coins, int amount, unordered_map<int,int>& mp){
        if(amount==0) return 0;
        if(mp.count(amount)) return mp[amount];
        int res=INT_MAX;
        for(auto coin:coins){
            if(coin>amount)
                continue;
            int rt=helper(coins,amount-coin,mp);
            if(rt==-1) continue;
            res=min(res,rt+1);
        }
        mp[amount]= res==INT_MAX? -1:res;
        return mp[amount];
    }
};

解法三（动态规划）：

class Solution {
public:
    int coinChange(vector<int>& coins, int amount) {
        vector<int> dp(amount + 1, INT_MAX);
        dp[0] = 0;
        for (int i = 1; i <= amount; i++) {
            for (auto coin : coins) {
                if (coin > i || dp[i - coin] == INT_MAX)
                    continue;
                dp[i] = min(dp[i], dp[i - coin] + 1);
            }
        }
        return dp[amount]==INT_MAX? -1:dp[amount];
    }
};