LeetCode-Combination Sum 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?

Credits:
Special thanks to @pbrother for adding this problem and creating all test cases.

public class Solution {
    public int combinationSum4(int[] nums, int target) {
        if (nums.length==0) return 0;
        Arrays.sort(nums);

        int[] combines = new int[target+1];
        combines[0] = 1;

        for (int val=1;val<=target;val++)
            for (int i=0;i<nums.length;i++)
                if (val >= nums[i]){
                    combines[val] += combines[val-nums[i]];
                } else break;

        return combines[target];
    }
}