I got ACC with an program running in O(N^3) like this:

    If sol[i][j]=minimum unhappiness that it is obtained with the
first i horses using j stables than sol[i][j]=min(sol[k][j-1]+
no[0]*no[1]) where k can take value between j-1 and i-1 and no[a]
represent the number of horses coloured with 'a' from the interval
[k+1, i]. You can use in this way O(N) memory and you can avoid
getting MLE. Good luck!