백준 - 알파벳 [1987]

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

vector<char> check;
vector<int> fin;
char arr[20][20];
int r, c;
bool flag = false;
bool visit[20][20];

void dfs(int i, int j, int cnt) {
	visit[i][j] = true;
	flag = false;
	if (i > 0 && !visit[i-1][j]) {
		for (int k = 0; k < (int)check.size(); k++)
			if (check[k] == arr[i - 1][j]) {
				flag = true;
				break;
			}
		if (!flag) {
			check.push_back(arr[i - 1][j]);
			dfs(i - 1, j, cnt+1);
		}
		flag = false;
	}
	if (j > 0 && !visit[i][j-1]) {
		for (int k = 0; k < (int)check.size(); k++)
			if (check[k] == arr[i][j - 1]) {
				flag = true;
				break;
			}
		if (!flag) {
			check.push_back(arr[i][j - 1]);
			dfs(i, j - 1, cnt + 1);
		}
		flag = false;
	}
	if (j < c - 1 && !visit[i][j+1]) {
		for (int k = 0; k < (int)check.size(); k++)
			if (check[k] == arr[i][j + 1]) {
				flag = true;
				break;
			}
		if (!flag) {
			check.push_back(arr[i][j + 1]);
			dfs(i, j + 1, cnt + 1);
		}
		flag = false;
	}
	if (i < r - 1 && !visit[i + 1][j]) {
		for (int k = 0; k < (int)check.size(); k++)
			if (check[k] == arr[i + 1][j]) {
				flag = true;
				break;
			}
		if (!flag) {
			check.push_back(arr[i + 1][j]);
			dfs(i + 1, j, cnt + 1);
		}
		flag = false;
	}
	if (cnt != 1)
		fin.push_back(cnt);
	visit[i][j] = false;
	check.pop_back();
}

int main() {

	cin >> r >> c;

	for (int i = 0; i < r; i++)
		for (int j = 0; j < c; j++)
			cin >> arr[i][j];

	check.push_back(arr[0][0]);
	dfs(0, 0, 1);

	sort(fin.begin(), fin.end(), greater<int>());

	cout << fin[0];

	return 0;
}