Leetcode 689. Maximum Sum of 3 Non-Overlapping Subarrays

Find three non-overlapping subarrays of length k whose total sum is maximized and return their starting indices (choose the lexicographically smallest tuple on ties). This is solved by computing k-length window sums and using prefix-sum/sliding-window + DP-like scans to track the best left and right intervals for each possible middle interval in linear time.