Create a count map for array $$$b$$$, say $$$cnt$$$. Then iterate over $$$a$$$ and for each $$$a_i$$$, $$$ans$$$ is increased by $$$cnt[a_i]$$$. cmiiw!

for(int i = 1; i <= b.size(); i++){
cnt[b[i]]++;
}

for(int i = 1; i <= a.size(); i++){
ans += cnt[a[i]];
}