Maximum Sum of Three Non-Overlapping Subarrays in Java (Time Complexity: O(n))

Given an array nums of integers, find three non-overlapping subarrays with maximum sum.

Return the total sum of the three subarrays

Example:

Input: [2, 3, -8, 7, -2, 9, -9, 7, -2, 4]
Output: 28
Explanation: Subarrays [2, 3], [7, -2, 9] and [7, -2, 4]
             have the maximum sum of 28

Note:

Subarrays must be non-empty
nums contains at least three numbers

Understanding the Problem

The core challenge of this problem is to find three non-overlapping subarrays that together have the maximum possible sum. This problem is significant in scenarios where we need to maximize the sum of multiple segments of data, such as in financial analysis or signal processing.

Potential pitfalls include overlapping subarrays and not considering all possible subarray combinations.

Approach

To solve this problem, we can break it down into manageable steps:

Calculate the sum of all possible subarrays of a given length.
Use dynamic programming to keep track of the maximum sums of subarrays ending at or starting from each index.
Combine these results to find the maximum sum of three non-overlapping subarrays.

Let's start with a naive solution and then optimize it.

Naive Solution

The naive solution involves checking all possible combinations of three non-overlapping subarrays. This approach is not optimal due to its high time complexity of O(n^3).

Optimized Solution

We can optimize the solution using dynamic programming. The idea is to precompute the maximum subarray sums for subarrays ending at or starting from each index, and then combine these results efficiently.

Algorithm

Here is a step-by-step breakdown of the optimized algorithm:

Compute the sum of all subarrays of length k.
Use two arrays to store the maximum subarray sums ending at and starting from each index.
Iterate through the array to find the maximum sum of three non-overlapping subarrays using the precomputed sums.

Code Implementation

public class MaxSumOfThreeSubarrays {
    public int maxSumOfThreeSubarrays(int[] nums, int k) {
        int n = nums.length;
        int[] sum = new int[n + 1];
        for (int i = 0; i < n; i++) {
            sum[i + 1] = sum[i] + nums[i];
        }

        int[] left = new int[n];
        int[] right = new int[n];
        int maxSum = 0;

        // Calculate max sum of subarray ending at each index
        for (int i = k - 1; i < n; i++) {
            int currentSum = sum[i + 1] - sum[i + 1 - k];
            if (currentSum > maxSum) {
                maxSum = currentSum;
                left[i] = i + 1 - k;
            } else {
                left[i] = left[i - 1];
            }
        }

        maxSum = 0;
        // Calculate max sum of subarray starting at each index
        for (int i = n - k; i >= 0; i--) {
            int currentSum = sum[i + k] - sum[i];
            if (currentSum >= maxSum) {
                maxSum = currentSum;
                right[i] = i;
            } else {
                right[i] = right[i + 1];
            }
        }

        int result = 0;
        // Find the maximum sum by combining the results
        for (int i = k; i <= n - 2 * k; i++) {
            int l = left[i - 1];
            int r = right[i + k];
            int total = (sum[l + k] - sum[l]) + (sum[i + k] - sum[i]) + (sum[r + k] - sum[r]);
            result = Math.max(result, total);
        }

        return result;
    }

    public static void main(String[] args) {
        MaxSumOfThreeSubarrays solution = new MaxSumOfThreeSubarrays();
        int[] nums = {2, 3, -8, 7, -2, 9, -9, 7, -2, 4};
        int k = 2;
        System.out.println(solution.maxSumOfThreeSubarrays(nums, k)); // Output: 28
    }
}

Complexity Analysis

The time complexity of this optimized solution is O(n), where n is the length of the input array. This is because we make a constant number of passes through the array. The space complexity is also O(n) due to the additional arrays used for storing sums and indices.

Edge Cases

Potential edge cases include:

Arrays with all negative numbers.
Arrays with all positive numbers.
Arrays with a mix of positive and negative numbers.

Each of these cases should be tested to ensure the algorithm handles them correctly.

Testing

To test the solution comprehensively, consider the following test cases:

Simple cases with small arrays.
Cases with large arrays to test performance.
Edge cases as mentioned above.

Using a testing framework like JUnit can help automate and manage these tests effectively.

Thinking and Problem-Solving Tips

When approaching such problems, it's essential to break them down into smaller parts and solve each part step-by-step. Practice solving similar problems and study different algorithms to improve problem-solving skills.

Conclusion

In this blog post, we discussed how to solve the problem of finding the maximum sum of three non-overlapping subarrays. We explored a naive solution and then optimized it using dynamic programming. Understanding and solving such problems is crucial for developing strong algorithmic skills.

Additional Resources

For further reading and practice, consider the following resources: