LeetCode-in-Java

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?

Solution

import java.util.Arrays;

public class Solution {
    private int[] storage;

    public int combinationSum4(int[] nums, int target) {
        storage = new int[target + 1];
        Arrays.fill(storage, -1);
        return result(nums, target);
    }

    private int result(int[] nums, int target) {
        if (target < 0) {
            return 0;
        }
        if (target == 0) {
            return 1;
        }
        if (storage[target] != -1) {
            return storage[target];
        }
        int count = 0;
        for (int i : nums) {
            count += result(nums, target - i);
        }
        storage[target] = count;
        return count;
    }
}

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