c++ sort() function isn't sorting ?

I'm tring to sort a subset of a vector with sort function . the problem is that the program compile perfectly but when debugging I noticed that sort function isn't actually sorting !!
so my question here is there anyway to detect if the sort function failed to sort the vector array string ..etc ? and how to solve such a problem .

#include <bits/stdc++.h>
#define FOR(i,j) for( int i = 0 ; i < j;i++)

using namespace std;
  int findMinDiff(vector<  int>      arr,   int b,  int e)
{
   // Sort vector in non-decreasing order
      sort(arr.begin()+b, arr.begin()+e);

   // Initialize difference as infinite
     int diff = INT_MAX;

   // Find the min diff by comparing adjacent
   // pairs in sorted vector
   for (vector<  int>::iterator i=arr.begin()+b; i!=arr.begin()+e-1; i++){
      if (*(i+1)-*i  < diff)
          diff =  *(i+1)-*i ;
   }

   // Return min diff
   return diff;
}


int main() {
      int  n, q;
    cin >> n >> q;
    vector<  int> a(n);
      int last=0;
    FOR(i,n){
        cin>> a[i];
    }
      int x,y;
      int l=0,r=0;
      int temp;
    FOR(i,q){
        cin>>x;
        cin>>y;
        l=1+(x+last)%n;
        r=1+(y+last)%n;
        if(l>r){
            temp=r;
            r=l;
            l=temp;
        }

        last=findMinDiff(a,l-1,r-1);
        cout<<"last: "<<last<<endl;

    }
    return 0;
}