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.

class Solution {
    public int combinationSum4(int[] nums, int target) {
        if (nums.length == 0) 
            return 0;
        Arrays.sort(nums);
        int[] dp = new int[target+1];
        dp[0] = 1;

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