Problem:
Score : 400 points

Problem Statement

Takahashi has a maze, which is a grid of H \times W squares with H horizontal rows and W vertical columns.

The square at the i-th row from the top and the j-th column is a "wall" square if S_{ij} is #, and a "road" square if S_{ij} is ..

From a road square, you can move to a horizontally or vertically adjacent road square.

You cannot move out of the maze, move to a wall square, or move diagonally.

Takahashi will choose a starting square and a goal square, which can be any road squares, and give the maze to Aoki.

Aoki will then travel from the starting square to the goal square, in the minimum number of moves required.

In this situation, find the maximum possible number of moves Aoki has to make.

Constraints

1 \leq H,W \leq 20

S_{ij} is . or #.

S contains at least two occurrences of ..

Any road square can be reached from any road square in zero or more moves.

Input

Input is given from Standard Input in the following format:

H W
S_{11}...S_{1W}
:
S_{H1}...S_{HW}

Output

Print the maximum possible number of moves Aoki has to make.

Sample Input 1

3 3
...
...
...

Sample Output 1

4

If Takahashi chooses the top-left square as the starting square and the bottom-right square as the goal square, Aoki has to make four moves.

Sample Input 2

3 5
...#.
.#.#.
.#...

Sample Output 2

10

If Takahashi chooses the bottom-left square as the starting square and the top-right square as the goal square, Aoki has to make ten moves.

Solution in Rust (1.42.0) (judge verdict: Accepted)

#![allow(unused_mut)]
#![allow(non_snake_case)]
#![allow(unused_variables)]
#![allow(unused_imports)]
#![allow(unused_macros)]
#![allow(dead_code)]
// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let mut $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };

    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}

macro_rules! max {
    ($x: expr) => ($x);
    ($x: expr, $($y: expr),+) => {
        cmp::max($x, max!($($y),+))
    };
}

macro_rules! min {
    ($x: expr) => ($x);
    ($x: expr, $($y: expr),+) => {
        cmp::min($x, min!($($y),+))
    };
}

macro_rules! abs {
    ($x:expr, $y:expr) => {
        cmp::max($x, $y) - cmp::min($x, $y)
    };
}

const MOD: usize = 1_000_000_000 + 7;

struct BinCoeff {
    size: usize,
    fac: Vec<usize>,
    finv: Vec<usize>,
    inv: Vec<usize>,
}

impl BinCoeff {
    fn new(size: usize) -> BinCoeff {
        let size = size + 1;
        let fac = vec![0; size];
        let finv = vec![0; size];
        let inv = vec![0; size];
        let mut bincoeff = BinCoeff {
            size,
            fac: fac,
            finv: finv,
            inv: inv,
        };
        bincoeff.com_init();
        bincoeff
    }

    fn com_init(&mut self) {
        let fac = &mut self.fac;
        let finv = &mut self.finv;
        let inv = &mut self.inv;
        let size = self.size;
        fac[0] = 1;
        fac[1] = 1;
        finv[0] = 1;
        finv[1] = 1;
        inv[1] = 1;
        for i in 2..size {
            fac[i] = fac[i - 1] * i % MOD;
            inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
            finv[i] = finv[i - 1] * inv[i] % MOD;
        }
    }

    fn bin_coeff(&mut self, n: usize, k: usize) -> usize {
        if k > n / 2 {
            self.bin_coeff(n, n - k)
        } else {
            self.fac[n] * (self.finv[k] * self.finv[n - k] % MOD) % MOD
        }
    }
}

struct UnionFind {
    par: Vec<usize>,
}

impl UnionFind {
    fn new(size: usize) -> UnionFind {
        UnionFind {
            par: (0..size).collect::<Vec<usize>>(),
        }
    }

    fn root(&mut self, node: usize) -> usize {
        let parent = self.par[node];
        if parent == node {
            parent
        } else {
            let parent = self.root(parent);
            self.par[node] = parent;
            parent
        }
    }

    fn unite(&mut self, a: usize, b: usize) {
        let root_a = self.root(a);
        let root_b = self.root(b);
        self.par[root_b] = root_a;
    }

    fn is_same(&mut self, a: usize, b: usize) -> bool {
        let root_a = self.root(a);
        let root_b = self.root(b);
        root_a == root_b
    }
}

// must input n <= m
fn gcd(n: usize, m: usize) -> usize {
    if n == 0 {
        m
    } else {
        gcd(m % n, n)
    }
}

fn printer<T, U>(mut array: T)
where
    T: Iterator<Item = U>,
    U: fmt::Display + Copy,
{
    if let Some(a) = array.next() {
        print!("{}", a);
    }
    for a in array {
        print!(" {}", a);
    }
    println!()
}

fn sieve_of_eratosthenes(max: usize) -> Vec<usize> {
    let mut candidates = vec![true; max + 1];
    candidates[0] = false;
    candidates[1] = false;
    let mut num = 2;
    for num in 2..=(max as f64).sqrt().floor() as usize {
        if candidates[num] {
            let prime = num;
            for not_prime in (2 * prime..=max).step_by(prime) {
                candidates[not_prime] = false;
            }
        }
    }
    candidates
        .iter()
        .enumerate()
        .filter(|&(_num, &is_p)| is_p)
        .map(|(num, _is_p)| num)
        .collect::<Vec<_>>()
}

fn factorize(num: usize) -> HashMap<usize, usize> {
    if num < 2 {
        return HashMap::new();
    }

    let mut remain = num;
    let mut factor = HashMap::new();
    let primes = sieve_of_eratosthenes((num as f64).sqrt().floor() as usize);
    for &p in primes.iter() {
        if remain < p {
            break;
        }
        if remain % p == 0 {
            let entry = factor.entry(p).or_insert(0);
            while remain % p == 0 {
                remain /= p;
                *entry += 1;
            }
        }
    }
    if remain != 1 {
        factor.insert(remain, 1);
    }
    factor
}

use std::collections::{BinaryHeap, HashMap, HashSet, VecDeque};
use std::io::{self, BufRead, Read};
use std::{cmp, fmt, iter, usize};

fn main() {
    input! {
        H: usize,
        W: usize,
        S: [chars; H],
    }
    let mut edges: HashMap<(usize, usize), HashSet<(usize, usize)>> = HashMap::with_capacity(H * W);
    let mut next = Vec::with_capacity(4);
    for x in 0..W {
        for y in 0..H {
            if S[y][x] == '.' {
                next.clear();
                if x + 1 < W {
                    next.push((x + 1, y));
                }
                if x > 0 {
                    next.push((x - 1, y));
                }
                if y + 1 < H {
                    next.push((x, y + 1));
                }
                if y > 0 {
                    next.push((x, y - 1));
                }
                for &(xx, yy) in next.iter() {
                    if S[yy][xx] == '.' {
                        edges
                            .entry((xx, yy))
                            .or_insert(HashSet::new())
                            .insert((x, y));
                        edges
                            .entry((x, y))
                            .or_insert(HashSet::new())
                            .insert((xx, yy));
                    }
                }
            }
        }
    }
    let mut ans = 0usize;
    let mut search_pool = VecDeque::with_capacity(H * W);
    let mut searched: HashSet<(usize, usize)> = HashSet::with_capacity(H * W);
    for &start in edges.keys() {
        search_pool.push_back((start, 0));
        searched.clear();
        searched.insert(start);
        while let Some((node, cost)) = search_pool.pop_front() {
            ans = max!(ans, cost);
            for &next in edges.get(&node).unwrap().iter() {
                if searched.insert(next) {
                    search_pool.push_back((next, cost + 1))
                }
            }
        }
    }
    println!("{}", ans);
}