思路：

dp。

實現：

 1 class Solution 
 2 {
 3 public:
 4     int combinationSum4(vector<int>& nums, int target) 
 5     {
 6         if (nums.empty()) return target == 0;
 7         vector<int> dp(target + 1, 0);
 8         dp[0] = 1;
 9         vector<int> tmp(nums.begin(), nums.end());
 
10         sort(tmp.begin(), tmp.end());
11         int n = tmp.size();
12         for (int i = 1; i <= target; i++)
13         {
14             for (int j = 0; tmp[j] <= i && j < n; j++)
15             {
16                 dp[i] += dp[i - tmp[j]];
17             }
18         }
19         return
 
 dp[target];
20     }
21 };

leetcode377 Combination Sum IV