PAT 1144 The Missing Number

Given N integers, you are supposed to find the smallest positive integer that is NOT in the given list.

Input Specification:
Each input file contains one test case. For each case, the first line gives a positive integer N (≤10^5). Then N integers are given in the next line, separated by spaces. All the numbers are in the range of int.

Output Specification:
Print in a line the smallest positive integer that is missing from the input list.

Sample Input:

10
5 -25 9 6 1 3 4 2 5 17

Sample Output:

7

#include<iostream>
#include<vector>
#include<set>
using namespace std;
int main(){
  int n, t, cnt=1;
  cin>>n;
  set<int> s;
  for(int i=0; i<n; i++){
    cin>>t;
    if(t>0)
      s.insert(t);
  }
  auto it=s.begin();
  for(; it!=s.end(); it++){
    if(cnt++!=*it)
      break;
  }
  it==s.end()?cout<<cnt<<endl:cout<<cnt-1<<endl;
  return 0;
}