LeetCode - Spiral Matrix (Java)

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].

Iterative Version:

public ArrayList<Integer> spiralOrder(int[][] matrix) { ArrayList<Integer> result = new ArrayList<Integer>();   if(matrix == null || matrix.length == 0) return result;   int m = matrix.length; int n = matrix[0].length;   int x=0; int y=0;   while(m>0 && n>0){   //if one row/column left, no circle can be formed if(m==1){ for(int i=0; i<n; i++){ result.add(matrix[x][y++]); } break; }else if(n==1){ for(int i=0; i<m; i++){ result.add(matrix[x++][y]); } break; }   //below, process a circle   //top - move right for(int i=0;i<n-1;i++){ result.add(matrix[x][y++]); }   //right - move down for(int i=0;i<m-1;i++){ result.add(matrix[x++][y]); }   //bottom - move left for(int i=0;i<n-1;i++){ result.add(matrix[x][y--]); }   //left - move up for(int i=0;i<m-1;i++){ result.add(matrix[x--][y]); }   x++; y++; m=m-2; n=n-2; }   return result; }

Recursive Version

public ArrayList<Integer> spiralOrder(int[][] matrix) {
        if(matrix==null || matrix.length==0) 
            return new ArrayList<Integer>();
 
        return spiralOrder(matrix,0,0,matrix.length,matrix[0].length);
    }
 
 
    public ArrayList<Integer> spiralOrder(int [][] matrix, int x, int y, int m, int n){
        ArrayList<Integer> result = new ArrayList<Integer>();
 
        if(m<=0||n<=0) 
            return result;
 
        //only one element left
        if(m==1&&n==1) {
            result.add(matrix[x][y]);
            return result;
        }
 
        //top - move right
        for(int i=0;i<n-1;i++){
            result.add(matrix[x][y++]);
        }
 
        //right - move down
        for(int i=0;i<m-1;i++){
            result.add(matrix[x++][y]);
        }
 
        //bottom - move left
        if(m>1){    
            for(int i=0;i<n-1;i++){
                result.add(matrix[x][y--]);
            }
        }
 
        //left - move up
        if(n>1){
            for(int i=0;i<m-1;i++){
                result.add(matrix[x--][y]);
            }
        }
 
        if(m==1||n==1)
            result.addAll(spiralOrder(matrix, x, y, 1, 1));
        else    
            result.addAll(spiralOrder(matrix, x+1, y+1, m-2, n-2));
 
        return result;
    }

Also refer to http://leetcode.com/2010/05/printing-matrix-in-spiral-order.html
Read full article from LeetCode – Spiral Matrix (Java)