Spiral Matrix

Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

For example,

Given the following matrix:

[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]

You should return [1,2,3,6,9,8,7,4,5].

The idea is to use recursion to print from outer loop to inner loop.

First Loop: top left[1,2] -> top right[3,6] -> bottom right[9,8] -> bottom left[7,4]

Second Loop: [5]

Use a level variable to track the loop level.

public ArrayList<Integer> spiralOrder(int[][] matrix) {
        ArrayList<Integer> results = new ArrayList<Integer>();
        if (matrix==null||matrix.length==0) {
            return results;
        }

        int rows = matrix.length;
        int columns = matrix[0].length;

        spiralOrder(results, matrix, rows, columns, 0);

        return results;
    }

    private void spiralOrder(ArrayList<Integer> results, int[][] matrix, int rows, int columns, int level) {
        //end recursion case
        if (rows <= 0 || columns <= 0) {
            return; 
        }
        // single row
        if (rows == 1) {
            for (int j = 0; j < columns; j++) {
                results.add(matrix[level][level+j]);
            }
            return;
         }
         if (columns == 1) {
            for (int i = 0; i < rows; i++) {
                results.add(matrix[level+i][level]);
            }
            return;
         }

         // print from top left
         for (int j = 0; j < columns - 1; j++) {
             results.add(matrix[level][level+j]);
         }
         // print from top right
         for (int i=0; i<rows-1; i++) {
             results.add(matrix[level+i][level+columns-1]);
         }
         // print from bottom right
         for (int j=0; j<columns-1; j++) {
             results.add(matrix[level+rows-1][level+columns-1-j]);
         }
         // print from bottom left
         for (int i=0; i<rows-1; i++) {
             results.add(matrix[level+rows-1-i][level]);
         }
         spiralOrder(results, matrix, rows-2, columns-2, level+1);
    }