Find number of closed islands

Given a binary matrix mat[][] of dimensions NxM such that 1 denotes land and 0 denotes water. Find the number of closed islands in the given matrix.

• Time : O(nm)
• Space : O(nm)

class Solution {
    void bfs(vector<vector<int>>& matrix, int sy, int sx, int n, int m) {
        int dy[4]{-1,0,1,0},dx[4]{0,1,0,-1};
        queue<pair<int,int>> q;
        matrix[sy][sx] = 0;
        q.push({sy,sx});
        while(!q.empty()) {
            auto p = q.front(); q.pop();
            auto y = p.first, x = p.second;
            for(int i = 0 ; i < 4 ; i++) {
                int ny = y + dy[i], nx = x + dx[i];
                if(0 <= ny and ny < n and 0 <= nx and nx < m and matrix[ny][nx] == 1) {
                    matrix[ny][nx] = 0;
                    q.push({ny,nx});
                }
            }
        }
    }
public:
    int closedIslands(vector<vector<int>>& matrix, int N, int M) {
        int res = 0;
        for(int i = 0; i < N; i++) {
            if(matrix[i][0] == 1) bfs(matrix, i, 0, N, M);
            if(matrix[i][M - 1] == 1) bfs(matrix, i, M - 1, N, M);
        }
        
        for(int i = 0; i < M; i++) {
            if(matrix[0][i] == 1) bfs(matrix,0,i,N,M);
            if(matrix[N-1][i] == 1) bfs(matrix,N-1,i,N,M);
        }
        
        for(int i = 0; i < N; i++) {
            for(int j = 0; j < M; j++) {
                if(matrix[i][j]) {
                    res++;
                    bfs(matrix,i,j,N,M);
                }
            }
        }
        return res;
    }
};