Remove Islands

• Time : O(nm)
• Space : O(n+m)

void bfs(vector<vector<int>>& matrix, int sy, int sx) {
	int n = matrix.size(), m = matrix[0].size();
	int dy[4]{-1,0,1,0},dx[4]{0,1,0,-1};
	queue<pair<int,int>> q;
	q.push({sy,sx});
	matrix[sy][sx] = -1;
	while(!q.empty()) {
		auto [y, x] = q.front(); q.pop();
		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] = -1;
				q.push({ny,nx});
			}
		}
	}
}
vector<vector<int>> removeIslands(vector<vector<int>> matrix) {
	int n = matrix.size(), m = matrix[0].size();
	for(int i = 0; i < n; i++) {
		if(matrix[i][0] == 1) bfs(matrix, i, 0);
		if(matrix[i][m-1] == 1) bfs(matrix, i, m-1);
	}
	for(int i = 0; i < m; i++) {
		if(matrix[0][i] == 1) bfs(matrix, 0, i);
		if(matrix[n-1][i] == 1) bfs(matrix, n-1, i);
	}
	for(int i = 0; i < n; i++) {
		for(int j = 0; j < m; j++) {
			if(matrix[i][j] == -1) matrix[i][j] = 1;
			else if(matrix[i][j] == 1) matrix[i][j] = 0;
		}
	}
	
	return matrix;
}