Can someone help with this?

why do greedy techniques work here?

The question link

Why not for you ?, Basically, after sorting both arrays, if we pick any two indices i and j , then min(a[i],b[i]) + min(a[j],b[j]) is greater than or equal to min(a[i],b[j]) + min(a[j],b[i]) .

You can try to convince yourself for the above inequality.