Leetcode 1000. Minimum Cost to Merge Stones

Given n piles where you can repeatedly merge exactly k consecutive piles at cost equal to their sum, compute the minimum total cost to merge all piles into one (or return -1 if impossible — possible only when (n-1) % (k-1) == 0). This is typically solved with interval dynamic programming that computes the minimum cost to reduce subarrays to a certain number of piles.