백준 1987 c++

DFS로 접근하여 풀긴 풀었는데 수행시간이 다른 C++정답자들의 2배가 나왔다. 어디서 문제가 생긴지 모르겠다...

#include <iostream>
#include <vector>

using namespace std;

vector <vector <char>> graph;
vector <bool> visited(30, false);//알파벳 방문 
vector<pair<int, int>>direction{ {1, 0}, {0, 1}, {-1, 0}, {0, -1} };
int R, C;
int max_count = 0;

void input_graph()
{
	int i, j;

	cin >> R >> C;
	graph.resize(R, vector<char>(C));
	for (i = 0; i < R; i++)
	{
		for (j = 0; j < C; j++)
		{
			cin >> graph[i][j];
		}
	}
	/*for (i = 0; i < R; i++)
	{
		for (j = 0; j < C; j++)
		{
			cout << graph[i][j];
		}
		cout << "\n";
	}*/

	return;
}

void DFS(int start_x, int start_y, int count)
{
	int i = 0;
	int next_x, next_y, next_index;
	int index = graph[start_y][start_x] - 'A';

	if (max_count < count)
	{
		max_count = count;
	}
	visited[index] = true;
	for (i = 0; i < direction.size(); i++)
	{
		next_x = start_x + direction[i].first;
		next_y = start_y + direction[i].second;
		if ((next_x >= 0 && next_x < C) && (next_y >= 0 && next_y < R))
		{
			next_index = graph[next_y][next_x] - 'A';
			if (visited[next_index] == false)
			{
				DFS(next_x, next_y, count + 1);
			}
		}
	}
	count--;
	visited[index] = false;

	return;
}

void find_graph()
{
	DFS(0, 0, 1);

	cout << max_count << "\n";
}

int main(void)
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);

	input_graph();
	find_graph();

	return 0;
}