Define the prefix sum arrays globally
public static int [ ] [ ] prefix_sum_row = new int [ 50 ] [ 51 ] ; public static int [ ] [ ] prefix_sum_col = new int [ 51 ] [ 50 ] ; public static boolean is_valid ( int r , int c , int size , int [ ] [ ] grid ) { int r_end = r + size , c_end = c + size ;