带限制的子序列和

// Java Stream API solution
// For this problem, the use of streams is not very efficient compared to the standard approach using deque,
// but we can still illustrate a functional approach. However, Java Stream API does not have direct support
// for efficiently solving this problem due to the need for maintaining a sliding window maximum.
// The solution provided uses imperative style because of the efficiency constraints.
// Here, we utilize the same deque method but just wrap it in stream-like operations.
import java.util.Deque;
import java.util.LinkedList;
import java.util.stream.IntStream;

public class Solution {
    public int constrainedSubsetSum(int[] nums, int k) {
        Deque<Integer> deque = new LinkedList<>();
        return IntStream.range(0, nums.length).map(i -> {
            if (!deque.isEmpty() && deque.peekFirst() < i - k) {
                deque.pollFirst();
            }
            int max = nums[i] + (deque.isEmpty() ? 0 : Math.max(0, nums[deque.peekFirst()]));
            while (!deque.isEmpty() && max > nums[deque.peekLast()]) {
                deque.pollLast();
            }
            deque.offerLast(i);
            return max;
        }).max().orElse(0);
    }
}