Choose two indices i j, then swap a[i] with a[j] and b[i] with b[j] simultaneously?

[qnhat2004]

You can swap the pair $(a_i, b_i)$ with $(a_j, b_j)$ simultanously using pair(is more optimistic) and map:

pair:

vector<pair<int, int>> v;
for (int i = 0; i < n; ++i)
   v.push_back({a[i], b[i]});
sort(v.begin(), v.end()); // Sort by first element (array a[])

// show array a[]
for (auto i : v) cout << i.first << ' ';
// show array b[]
for (auto i : v) cout << i.second<< ' ';

map:

map<int, int> mp;

for (int i = 0; i < n; ++i)
   mp[a[i]] = b[i]; // Mapping the value of a[i] to b[i]

// Change order:
sort(a, a + n);

// show two array after changing order:
for (int i = 0; i < n; ++i) 
   cout << a[i] << ' '; // array A
for (int i = 0; i < n; ++i) 
   cout << mp[a[i]] << ' '; // array B after change the order with each a[i]