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 Rust (1.42.0) (judge verdict: Accepted)

use std::collections::HashSet;
use std::error::Error;
use std::io;

pub fn main() {
    if let Err(_e) = run() {
        panic!("failed.");
    }
}

fn run() -> Result<(), Box<dyn Error>> {
    let mut input = String::new();
    io::stdin().read_line(&mut input)?;
    let nums: Vec<i64> = input
        .trim()
        .split_whitespace()
        .map(|s| s.parse::<i64>().unwrap())
        .collect();
    let x = nums[0];
    let n = nums[1];
    input.clear();

    io::stdin().read_line(&mut input)?;
    let mut ps: HashSet<i64> = input
        .trim()
        .split_whitespace()
        .map(|s| s.parse::<i64>().unwrap())
        .collect();

    if !ps.contains(&x) || n == 0 {
        println!("{}", x);
        return Ok(());
    }

    let mut all = Vec::new();
    for i in 0..=101 {
        if !ps.contains(&i) || i == x {
            all.push(i);
        }
    }
    let index = match all.binary_search(&x) {
        Ok(index) => index,
        _ => panic!(""),
    };
    if all[index + 1] - x >= x - all[index - 1] {
        println!("{}", all[index - 1]);
    } else {
        println!("{}", all[index + 1]);
    }
    Ok(())
}