A greedy approach will not work as the examples show.

Try all possible moves using DP.

For the DP state, dp[i][j] is the minimum time cost to build the first i blocks using j workers.

In one step you can either assign a worker to a block or choose a number of workers to split.

If you choose to assign a worker to a block it is always better to assign him to the block with the maximum time so we sort the array before using DP.

To optimize the solution from O(n^3) to O(n^2) notice that if you choose to split, it is always better to split all the workers you have.