dfs

C++ 代码

#include <iostream>
#include <unordered_set>
using namespace std;
const int N = 22;
char d[N][N];
int n, m, ret = 0;
unordered_set<char> mm;

int dxy[] = { 0, 1, 0, -1, 0 };

void dfs(int x, int y) 
{
    ret = max(ret, (int)mm.size());
    for(int i = 0; i < 4; ++ i){
        int nx = x + dxy[i], ny = y + dxy[i + 1];
        if(nx >= 0 && nx < n && ny >= 0 && ny < m && mm.count(d[nx][ny]) == 0)
        {
            mm.insert(d[nx][ny]);
            dfs(nx, ny);
            mm.erase(d[nx][ny]);
        }    
    }
}

int main()
{
    cin >> n >> m;
    for(int i = 0; i < n; ++ i)
        for(int j = 0; j < m; ++ j)
            cin >> d[i][j];

    mm.insert(d[0][0]);
    dfs(0, 0);

    cout << ret << endl;

    return 0;
}