Revise paste Untitled

#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
typedef long long ll;
const int maxn = 5000;

vector<int> graph[maxn];

int main() {
    int n, m;
    cin >> n >> m;
    
    char c[n][m];
    
    
    int mat[n][m];
    int idx = 0;
    
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < m; j++) {
            cin >> c[i][j];
            mat[i][j] = idx;
            idx++;
        }
    }
    
    int di[] = {-1, 1, 0, 0};
    int dj[] = {0, 0, -1, 1};
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < m; j++) {
            for(int k = 0; k < 4; k++) {
                int ti = i + di[k];
                int tj = j + dj[k];
                
                if(ti >= 0 and ti < n and tj >= 0 and tj < m and c[i][j] == c[ti][tj]) {
                    graph[mat[i][j]].push_back(mat[ti][tj]);
                    graph[mat[ti][tj]].push_back(mat[i][j]);
                    
                }
            }
        }
    }
    
    
    
    return 0;
}