Language: C++

#include<bits/stdc++.h>
using ll = long long;
using namespace std;
int main(){
   	string N;
    cin >> N;
  	string ans = "";
  	sort(N.begin(),N.end());
  	ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
  	for(ll i = 0; i < N.size(); ++i){
     	if(N[i] != table[i]) ans = table[i];
    }
  	if(ans = "") cout << "None" << endl;
    else cout << ans << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:9:25: error: 'a' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                         ^
a.cc:9:27: error: 'b' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                           ^
a.cc:9:29: error: 'c' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                             ^
a.cc:9:31: error: 'd' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                               ^
a.cc:9:33: error: 'e' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                 ^
a.cc:9:35: error: 'f' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                   ^
a.cc:9:37: error: 'g' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                     ^
a.cc:9:39: error: 'h' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                       ^
a.cc:9:41: error: 'i' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                         ^
a.cc:9:43: error: 'j' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                           ^
a.cc:9:45: error: 'k' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                             ^
a.cc:9:47: error: 'l' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                               ^
a.cc:9:49: error: 'm' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                 ^
a.cc:9:51: error: 'n' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                   ^
a.cc:9:53: error: 'o' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                     ^
a.cc:9:55: error: 'p' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                       ^
a.cc:9:57: error: 'q' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                         ^
a.cc:9:59: error: 'r' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                           ^
a.cc:9:61: error: 's' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                             ^
a.cc:9:63: error: 't' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                               ^
a.cc:9:65: error: 'u' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                                 ^
a.cc:9:67: error: 'v' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                                   ^
a.cc:9:69: error: 'w' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                                     ^
a.cc:9:71: error: 'x' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                                       ^
a.cc:9:73: error: 'y' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                                         ^
a.cc:9:75: error: 'z' was not declared in this scope
    9 |         ll table[26] = {a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z};
      |                                                                           ^
a.cc:13:16: error: could not convert 'ans.std::__cxx11::basic_string<char>::operator=(((const char*)""))' from 'std::__cxx11::basic_string<char>' to 'bool'
   13 |         if(ans = "") cout << "None" << endl;
      |            ~~~~^~~~
      |                |
      |                std::__cxx11::basic_string<char>