CheckOneToOne

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

//O(n^2)
bool checkOneOne(const vector<int> &a){
bool check = true;
for(size_t i=0;i<a.size()-1;i++){
    for(size_t j=i+1;j<a.size();j++){
          if(a[i]==a[j]) {
                check=false;
                break;
          }
    }
}
return check;
}

//O(n)
bool checkOneOne2(const vector<int> &a){
vector<int> b;
b.resize(a.size());
bool check = true;
for(size_t i=0;i<a.size()-1;i++){
    if(b[a[i]]==0){
        b[a[i]]++;
    }
    else {
        check=false;
    }
}
return check;
}

int main() {
  vector<int> v;
  int n;
  cin >> n;
  v.resize(n);
  for (int i = 0;i < n;i++) {
    cin >> v[i]; // guarantee 0 ... n-1
  }

  //print "YES" only when v[i] is 1-1 and onto on set {0..n-1}
  //print "NO" otherwise
cout<<(checkOneOne2(v)?"YES" : "NO")<<endl;



}