# 377. Combination Sum IV

Medium
Given an array of distinct integers `nums` and a target integer `target`, return the number of possible combinations that add up to `target`.
The test cases are generated so that the answer can fit in a 32-bit integer.
Example 1:
Input: nums = [1,2,3], target = 4
Output: 7
Explanation:
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.
Example 2:
Input: nums = , target = 3
Output: 0
Constraints:
• `1 <= nums.length <= 200`
• `1 <= nums[i] <= 1000`
• All the elements of `nums` are unique.
• `1 <= target <= 1000`
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?

### 解題

Runtime: 0 ms, faster than 100%
Memory Usage: 2 MB, less than 84.21%
func combinationSum4(nums []int, target int) int {
dp := make([]int, target + 1)
dp = 0
for _, num := range nums {
if num <= target {
dp[num] = 1
}
}
for i:=1; i<=target; i++ {
for _, num := range nums {
if i + num <= target {
dp[i + num] += dp[i]
}
}
}
return dp[target]
}