River Sizes Poblem

Program.java

import java.util.*;

class Program {
	public static List<Integer> riverSizes(int[][] matrix) {
		var sizes = new ArrayList<Integer>();
		for(var i=0; i < matrix.length; i++) {
			for (var j=0; j < matrix[i].length; j++) {
				var size = size(i, j, matrix);
				if (size > 0) {
					sizes.add(size);
				}
			}
		}
    return sizes;
  }
	private static int size(int i, int j, int[][] matrix) {
		if (i < 0 || i >= matrix.length || 
			  j < 0 || j >= matrix[i].length || 
			  matrix[i][j] == 0) {
			return 0;
		}

		matrix[i][j] = 0;
		
		return 1 +
					 size(i + 1, j, 		matrix) +
					 size(i - 1, j,     matrix) +
					 size(i   ,  j + 1, matrix) + 
					 size(i   ,  j - 1, matrix); 
	}
}