【LeetCode每日一题】377. 组合总和 Ⅳ

输入：nums = [1,2,3], target = 4
输出：7
解释：
所有可能的组合为：
(1, 1, 1, 1)
(1, 1, 2)
(1, 2, 1)
(1, 3)
(2, 1, 1)
(2, 2)
(3, 1)
请注意，顺序不同的序列被视作不同的组合。

class Solution {
public:
    vector<int> memo;
    int combinationSum4(vector<int>& nums, int target) {
        int n = nums.size();
        memo = vector<int>(target + 1, -1);
        return dfs(nums, target);
    }

    int dfs(vector<int>& nums, int target) {
        if (target == 0) return memo[target] = 1;
        if (memo[target] != -1) return memo[target];
        int res = 0;
        
        for (auto num : nums) {
            if (target - num >= 0) 
                res += dfs(nums, target - num);
        }

        return memo[target] = res;
    }
};

class Solution {
public:
    int combinationSum4(vector<int>& nums, int target) {
        vector<unsigned long long> dp(target + 1, 0);
        dp[0] = 1;
        for (int i = 1; i <= target; i++) {
            for (auto num : nums) {
                if (i - num >= 0) {
                    dp[i] += dp[i - num];
                }
            }
        }
        return dp[target];
    }
};