377 Combination Sum IV 组合之和 IV

Given an integer array with all positive numbers and no duplicates, find the number of possible combinations that add up to a positive integer target.
Example:
nums = [1, 2, 3]
target = 4
The possible combination ways are:
(1, 1, 1, 1)
(1, 1, 2)
(1, 2, 1)
(1, 3)
(2, 1, 1)
(2, 2)
(3, 1)
Note that different sequences are counted as different combinations.
Therefore the output is 7.

Follow up:
What if negative numbers are allowed in the given array?
How does it change the problem?
What limitation we need to add to the question to allow negative numbers?

详见：https://leetcode.com/problems/combination-sum-iv/description/

C++：

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

参考：https://www.cnblogs.com/grandyang/p/5705750.html