Leetcode 54 Spiral matrix - Study java code

Leetcode54_SpiralMatrix_JavaStudy.java

public class Solution {
    public ArrayList<Integer> spiralOrder(int[][] matrix) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(matrix==null || matrix.length==0 || matrix[0].length==0) return new ArrayList<Integer>();
        return spiralOrder(matrix,0,0,matrix.length,matrix[0].length);
    }
// June 11, 2015 Julia likes the implementation documented in the blog. 
// She chose to study the algorithm. At that time, she did not know that 
// Leetcode has discussion panel, and there are hundreds of ideas in the 
// discussion panel
// this one is recursive solution, base cases are clearly presented 
// at the beginning of the function
//http://gongxuns.blogspot.ca/2012/12/leetcode-spiral-matrix.html        
    public ArrayList<Integer> spiralOrder(int [][] matrix, int x, int y, int m, int n){
        ArrayList<Integer> res = new ArrayList<Integer>();
        
        // base cases:
        if(m <= 0|| n <= 0) return res;
        
        // one element
        if(m ==1 && n == 1) {
            res.add(matrix[x][y]);
            return res;
        }
      
        // one row
        for(int i=0;i<n-1;i++){
            res.add(matrix[x][y++]);
        }
      
        // one column
        for(int i=0;i<m-1;i++){
            res.add(matrix[x++][y]);
        }
        
        if( m>1 ){    
            for(int i=0;i<n-1;i++){
                res.add(matrix[x][y--]);
            }
        }
        
        if(n>1){
            for(int i=0;i<m-1;i++){
                res.add(matrix[x--][y]);
            }
        }
        
        if( m==1 || n==1)
            res.addAll(spiralOrder(matrix,x,y,1,1));
        else    
            res.addAll(spiralOrder(matrix,x+1,y+1,m-2,n-2));
      
        return res;
    }
}