Language: C++

#include <bits/stdc++.h>
#include<math.h>
#include <iostream>
#include <vector>
#include <queue>

using namespace std;

int main() {
  string s;
  cin>>s;
  vector<vector<char>> al(2, vector<char>(26));
  al.at(0).at(0)='a';
  al.at(1).at(0)='b';
  al.at(2).at(0)='c';
  al.at(3).at(0)='d';
  al.at(4).at(0)='e';
  al.at(5).at(0)='f';
  al.at(6).at(0)='g';
  al.at(7).at(0)='h';
  al.at(8).at(0)='i';
  al.at(9).at(0)='j';
  al.at(10).at(0)='k';
  al.at(11).at(0)='l';
  al.at(12).at(0)='m';
  al.at(13).at(0)='n';
  al.at(14).at(0)='o';
  al.at(15).at(0)='p';
  al.at(16).at(0)='q';
  al.at(17).at(0)='r';
  al.at(18).at(0)='s';
  al.at(19).at(0)='t';
  al.at(20).at(0)='u';
  al.at(21).at(0)='v';
  al.at(22).at(0)='w';
  al.at(23).at(0)='x';
  al.at(24).at(0)='y';
  al.at(25).at(0)='z';
  
  for(int i=0; i<s.size();i++){
    
    if(s.at(i)=='a'){
      al.at(0).at(1)=1;
    }
    if else(s.at(i)=='b'){
      al.at(1).at(1)=1;
    }
    if else(s.at(i)=='c'){
      al.at(2).at(1)=1;
    }
    if else(s.at(i)=='d'){
      al.at(3).at(1)=1;
    }
    if else(s.at(i)=='e'){
      al.at(4).at(1)=1;
    }
    if else(s.at(i)=='f'){
      al.at(5).at(1)=1;
    }
    if else(s.at(i)=='g'){
      al.at(6).at(1)=1;
    }
    if else(s.at(i)=='h'){
      al.at(7).at(1)=1;
    }
    if else(s.at(i)=='i'){
      al.at(8).at(1)=1;
    }
    if else(s.at(i)=='j'){
      al.at(9).at(1)=1;
    }
    if else(s.at(i)=='k'){
      al.at(10).at(1)=1;
    }
    if else(s.at(i)=='l'){
      al.at(11).at(1)=1;
    }
    if else(s.at(i)=='m'){
      al.at(12).at(1)=1;
    }
    if else(s.at(i)=='n'){
      al.at(13).at(1)=1;
    }
    if else(s.at(i)=='o'){
      al.at(14).at(1)=1;
    }
    if else(s.at(i)=='p'){
      al.at(15).at(1)=1;
    }
    if else(s.at(i)=='q'){
      al.at(16).at(1)=1;
    }
    if else(s.at(i)=='r'){
      al.at(17).at(1)=1;
    }
    if else(s.at(i)=='s'){
      al.at(18).at(1)=1;
    }
    if else(s.at(i)=='t'){
      al.at(19).at(1)=1;
    }
    if else(s.at(i)=='u'){
      al.at(20).at(1)=1;
    }
    if else(s.at(i)=='v'){
      al.at(21).at(1)=1;
    }
    if else(s.at(i)=='w'){
      al.at(22).at(1)=1;
    }
    if else(s.at(i)=='x'){
      al.at(23).at(1)=1;
    }
    if else(s.at(i)=='y'){
      al.at(24).at(1)=1;
    }
    else{
      al.at(25).at(1)=1;
    }
  }
  bool a=true;
  for(int i=0; i<26;i++){
    if(al.at(i).at(1)==0){
      cout<<al.at(i).at(0)<<endl;
      a=false;
      break;
    }
  }
  if(a){
    cout<<"NONE"<<endl;
  }
    
}



Compiler output:
a.cc: In function 'int main()':
a.cc:45:8: error: expected '(' before 'else'
   45 |     if else(s.at(i)=='b'){
      |        ^~~~
      |        (
a.cc:48:8: error: expected '(' before 'else'
   48 |     if else(s.at(i)=='c'){
      |        ^~~~
      |        (
a.cc:51:8: error: expected '(' before 'else'
   51 |     if else(s.at(i)=='d'){
      |        ^~~~
      |        (
a.cc:54:8: error: expected '(' before 'else'
   54 |     if else(s.at(i)=='e'){
      |        ^~~~
      |        (
a.cc:57:8: error: expected '(' before 'else'
   57 |     if else(s.at(i)=='f'){
      |        ^~~~
      |        (
a.cc:60:8: error: expected '(' before 'else'
   60 |     if else(s.at(i)=='g'){
      |        ^~~~
      |        (
a.cc:63:8: error: expected '(' before 'else'
   63 |     if else(s.at(i)=='h'){
      |        ^~~~
      |        (
a.cc:66:8: error: expected '(' before 'else'
   66 |     if else(s.at(i)=='i'){
      |        ^~~~
      |        (
a.cc:69:8: error: expected '(' before 'else'
   69 |     if else(s.at(i)=='j'){
      |        ^~~~
      |        (
a.cc:72:8: error: expected '(' before 'else'
   72 |     if else(s.at(i)=='k'){
      |        ^~~~
      |        (
a.cc:75:8: error: expected '(' before 'else'
   75 |     if else(s.at(i)=='l'){
      |        ^~~~
      |        (
a.cc:78:8: error: expected '(' before 'else'
   78 |     if else(s.at(i)=='m'){
      |        ^~~~
      |        (
a.cc:81:8: error: expected '(' before 'else'
   81 |     if else(s.at(i)=='n'){
      |        ^~~~
      |        (
a.cc:84:8: error: expected '(' before 'else'
   84 |     if else(s.at(i)=='o'){
      |        ^~~~
      |        (
a.cc:87:8: error: expected '(' before 'else'
   87 |     if else(s.at(i)=='p'){
      |        ^~~~
      |        (
a.cc:90:8: error: expected '(' before 'else'
   90 |     if else(s.at(i)=='q'){
      |        ^~~~
      |        (
a.cc:93:8: error: expected '(' before 'else'
   93 |     if else(s.at(i)=='r'){
      |        ^~~~
      |        (
a.cc:96:8: error: expected '(' before 'else'
   96 |     if else(s.at(i)=='s'){
      |        ^~~~
      |        (
a.cc:99:8: error: expected '(' before 'else'
   99 |     if else(s.at(i)=='t'){
      |        ^~~~
      |        (
a.cc:102:8: error: expected '(' before 'else'
  102 |     if else(s.at(i)=='u'){
      |        ^~~~
      |        (
a.cc:105:8: error: expected '(' before 'else'
  105 |     if else(s.at(i)=='v'){
      |        ^~~~
      |        (
a.cc:108:8: error: expected '(' before 'else'
  108 |     if else(s.at(i)=='w'){
      |        ^~~~
      |        (
a.cc:111:8: error: expected '(' before 'else'
  111 |     if else(s.at(i)=='x'){
      |        ^~~~
      |        (
a.cc:114:8: error: expected '(' before 'else'
  114 |     if else(s.at(i)=='y'){
      |        ^~~~
      |        (
a.cc:117:5: error: 'else' without a previous 'if'
  117 |     else{
      |     ^~~~