Unique K-Number Sum

JavaView on GFG

Time: O(n)

Space: O(k)

Intuition

Find sum of unique k-length subarrays or sum of elements appearing exactly once.

Algorithm

1Count frequency of each element. Sum elements with frequency == 1.

Common Pitfalls

•Frequency map. O(n). Sum only elements that appear exactly once in the array/substring.

Unique K-Number Sum.java

Java

// Approach: Sliding window of size k with frequency map. Sum unique values in each window.
// Time: O(n) Space: O(k)
import java.util.*;

class Solution {
    public ArrayList<ArrayList<Integer>> combinationSum(int n, int k) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        backtrack(1, new ArrayList<>(), 0, k, n, result);
        return result;
    }

    private void backtrack(int start, ArrayList<Integer> current, int sum, int k, int n,
            ArrayList<ArrayList<Integer>> result) {
        if (current.size() == k) {
            if (sum == n)
                result.add(new ArrayList<>(current));
            return;
        }

        for (int i = start; i <= 9; i++) {
            if (sum + i > n)
                break; // Prune the branch if sum exceeds n

            current.add(i);
            backtrack(i + 1, current, sum + i, k, n, result);
            current.remove(current.size() - 1);
        }
    }
}

Was this solution helpful?