推荐DFS

#include <iostream>
using namespace std;
const int N = 30;
char graph[N][N];
bool vis[N][N];
bool letter[26];
int n, m;
int res;
int dic[4][2] = {
    {0, -1}, {-1, 0}, {0, 1}, {1, 0}
};
bool check(int x, int y){
    if(x >= 0 && x < n && y >= 0 && y < m)  return true;
    return false;
}
void dfs(int x, int y, int cnt){
    if(cnt > res)   res = cnt;
    for(int i = 0; i < 4; i ++ ){
        int xx = x + dic[i][0], yy = y + dic[i][1];
        if(check(xx, yy) && !vis[xx][yy] && !letter[graph[xx][yy] - 'A']){
            vis[xx][yy] = true;
            letter[graph[xx][yy] - 'A'] = true;
            dfs(xx, yy, cnt + 1);
            letter[graph[xx][yy] - 'A'] = false;
            vis[xx][yy] = false;
        }
    }
}
int main(){
    cin >> n >> m;
    for(int i = 0; i < n; i ++ )    cin >> graph[i];
    letter[graph[0][0] - 'A'] = true;
    dfs(0, 0, 1);
    cout << res << endl;
    return 0;
}

BFS

#include <iostream>
#include <queue>
#include <unordered_map>
#include <cstring>
using namespace std;
typedef pair<int, int> pii;
const int N = 25;
char graph[N][N];
int n, m;
struct node{
    pii loc;
    bool flag[26];
    int cnt;
};
int dic[4][2] = {
    {0, -1}, {-1, 0}, {0, 1}, {1, 0}
};
bool check(int x, int y){
    if(x >= 0 && x < n && y >= 0 && y < m) return true;
    return false;
}
int res;
void bfs(){
    queue<node> qu;
    node st;
    st.loc = {0, 0}, st.flag[graph[0][0] - 'A'] = true;
    st.cnt = 1;
    qu.push(st);
    while(!qu.empty()){
        node now = qu.front();
        qu.pop();
        res = max(res, now.cnt);
        for(int i = 0; i < 4; i ++ ){
            int xx = now.loc.first + dic[i][0];
            int yy = now.loc.second + dic[i][1];
            if(check(xx, yy) && !now.flag[graph[xx][yy] - 'A']){
                node nxt;
                nxt.loc = {xx, yy}, nxt.cnt = now.cnt;
                memcpy(nxt.flag, now.flag, sizeof now.flag);
                nxt.cnt ++ ;
                nxt.flag[graph[xx][yy] - 'A'] = true;
                qu.push(nxt);
            }
        }
    }
}
int main(){
    cin >> n >> m;
    for(int i = 0; i < n; i ++ )    cin >> graph[i];
    bfs();
    cout << res << endl;
    return 0;
}