😕377 combination sum IV

https://leetcode.com/problems/combination-sum-iv/solution/

class Solution {
    public int combinationSum4(int[] nums, int target) {
        int[] dp = new int[target + 1];
        dp[0] = 1;
        for (int sum = 0; sum <= target; sum++) {
            for (int num : nums) {
                if (num <= sum) {
                    // dp[sum - num]的值一定比dp[sum]先确定
                    dp[sum] += dp[sum - num];
                }
            }
        }
        return dp[target];
    }
}

这个dp有点东西，我不是很懂它怎么保证一个数可以取多次的