Buying Gift

 #include <bits/stdc++.h>

 using namespace std;

 typedef long long ll;

 struct Department{
     ll a, b;
     bool operator<(const Department& v) const{
         return a<v.a;
     }
};

 int t;

 int main(){
     //freopen("buyinggifts.in", "r", stdin);

     cin >> t;
     while(t--){
         int n;
         cin >> n;
         vector<Department> dep(n);
         vector<ll> mx(n, -1);
         for(int i=0; i<n; i++){
             cin >> dep[i].a >> dep[i].b;
         }
         sort(dep.begin(), dep.end());
         ll mxv = -1;
         //!!!Need to use base values as -1 to check with right values at line 41 because there are some
         //values that are possibly equal to 0;
         for(int i=n-1; i>=0; i--){
             mx[i] = mxv;
             mxv = max(mxv, dep[i].b);
             //Get suffix to compare later on in calculation
         }
         set<ll> works;
         ll ans = LLONG_MAX;
         for(int i=0; i<n; i++){
             if(i<n-1){
                 ans = min(ans, abs(mx[i]-dep[i].a));
                 //check with max values because max values are always valid
             }
             if(dep[i].a>mx[i] && !works.empty()){
                 auto it = works.upper_bound(dep[i].a);
                 //get upper_bound and check out of bounds to see which values work under top condition
                 //on line 44
                 if(it!=works.end()){
                     ans = min(ans, abs(*it-dep[i].a));
                 }
                 //it--;
                 if(it!=works.begin()){
                     it--;
                     ans = min(ans, abs(*it-dep[i].a));
                 }
             }
             works.insert(dep[i].b);
             //Keep values that can be valid to be used
         }
         cout << ans << '\n';
     }

     return 0;
}