black shapes interview bit (using c++ lambda for recursive dfs)

Black_Shapes.cpp

int n;
int m;
int dx[4] = {-1, 0, 1, 0};
int dy[4] = {0, 1, 0, -1};

int Solution::black(vector<string> &A) {
    n = A.size();
    if (n == 0) return 0;
    m = A[0].length();
    vector < vector < bool > > visited(n, vector < bool > (m, false));
    
    function < void (int, int) > dfs;
    dfs = [&](int u, int v) -> void {
        visited[u][v] = true;
        
        for (int i = 0; i < 4; i++) {
            int pp = u + dx[i];
            int qq = v + dy[i];
            
            if (pp >= 0 && qq >= 0 && pp < n && qq < m)
                if (!visited[pp][qq] && A[pp][qq] == 'X')
                    dfs(pp, qq);
        }
    };
    
    int ans = 0;
    for (int i = 0; i < n; i++)
        for (int j = 0; j < m; j++) {
            if (!visited[i][j] && A[i][j] == 'X') {
                ans++;
                dfs(i, j);
            }
        }
    
    return ans;
}