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 = [9], 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] = 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]
}