Problem:
Score : 300 points

Problem Statement

Given are an integer X and an integer sequence of length N: p_1, \ldots, p_N.

Among the integers not contained in the sequence p_1, \ldots, p_N (not necessarily positive), find the integer nearest to X, that is, find the integer whose absolute difference with X is the minimum. If there are multiple such integers, report the smallest such integer.

Constraints

1 \leq X \leq 100

0 \leq N \leq 100

1 \leq p_i \leq 100

p_1, \ldots, p_N are all distinct.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

X N
p_1 ... p_N

Output

Print the answer.

Sample Input 1

6 5
4 7 10 6 5

Sample Output 1

8

Among the integers not contained in the sequence 4, 7, 10, 6, 5, the one nearest to 6 is 8.

Sample Input 2

10 5
4 7 10 6 5

Sample Output 2

9

Among the integers not contained in the sequence 4, 7, 10, 6, 5, the ones nearest to 10 are 9 and 11. We should print the smaller one, 9.

Sample Input 3

100 0

Sample Output 3

100

When N = 0, the second line in the input will be empty. Also, as seen here, X itself can be the answer.

Solution in C++ (GCC 9.2.1) (judge verdict: Wrong Answer)

#include <iostream>
#include <algorithm>
#include <vector>
#include <tuple>
using namespace std;

int main()
{
  int x, n;

  cin >> x >> n;

  if(n == 0){
    cout << x << endl;
    return 0;
  }

  vector<int> vec(n);
  //vector<tuple<int, int>> tmp(n);
  vector<int> upper;
  vector<int> lower;

  for(int i=0; i<n; ++i){
    cin >> vec[i];
    //tmp[i] = tuple<int,int>(abs(x-vec[i]), vec[i]);

    if(vec[i] <= x){
      lower.push_back(vec[i]);
    }
    else if(vec[i] > x){
      upper.push_back(vec[i]);
    }
  }

  sort(upper.begin(), upper.end(), [&x](int& a, int& b){return abs(x-a)<abs(x-b);});
  sort(lower.begin(), lower.end(), [&x](int& a, int& b){return abs(x-a)<abs(x-b);});
/*
  for(int e : upper){
    cout << e << ' ';
  }
  cout << endl;

  for(int e : lower){
    cout << e << ' ';
  }
  cout << endl;
*/
  int a = x;
  int b = x+1;

  for(int i=0; i<n; ++i){
    if(i<lower.size() && a!=lower[i]){
      cout << a << endl;
      return 0;
    }
    else --a;

    if(i<upper.size() && b!=upper[i]){
      cout << b << endl;
      return 0;
    }
    else ++b;
  }

  cout << lower.back()-1 << endl;

  /*
  int a = x;

  for(size_t i=0; i<lower.size(); ++i){
    if(a != lower[i]){
      cout << a << endl;
      return 0;
    }
    else{
      --a;
    }
  }

  a = x+1;

  for(size_t i=0; i<upper.size(); ++i){
    if(a != upper[i]){
      cout << a << endl;
      return 0;
    }
    else{
      ++a;
    }
  }

  cout << lower.back()-1 << endl;
  */
  return 0;
}