Is it good to use unordered map for every dp problem as it could be more memory efficient in comparison to vector in c++

https://mirror.codeforces.com/blog/entry/129992 in this editorial , the editorial of c, what if the no bad exist, like n = 2, m=2; a == (3 3 5 5 1) b == (5 5 3 3 2) i will not consider the 1,2 as it is at the m+n+1 index so before that no bad exist, am i wrong ?

https://mirror.codeforces.com/problemset/problem/1831/B https://mirror.codeforces.com/contest/1831/submission/262171365 is my submission