54. Spiral Matrix

https://leetcode.com/problems/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].

http://www.programcreek.com/2013/01/leetcode-spiral-matrix-java/

public class Solution {
    public List<Integer> spiralOrder(int[][] matrix) {
         List<Integer> result = new ArrayList<Integer>();
        if (matrix==null || matrix.length == 0 || matrix[0].length == 0) { 
            return result;
        }
 
        int m = matrix.length;
        int n = matrix[0].length;
        int left = 0;
        int right=n - 1;
        int top = 0;
        int bottom = m - 1;
        
        while (result.size() < m * n) {
            for (int i = left; i <= right; i++) {
                result.add(matrix[top][i]);
            }
            
            top++;
            
            for (int i = top; i <= bottom; i++) {
                result.add(matrix[i][right]);
            }
            
            right--;
            
            // prevent duplicate row
            if (bottom < top) {
                break;
            }
            
            for (int i = right; i >= left; i--) {
                result.add(matrix[bottom][i]);
            }
            
            bottom--;
            
            // prevent duplicate column
            if (right < left) {
                break;
            }
            
            for (int i = bottom; i >= top; i--) {
                result.add(matrix[i][left]);
            }
            
            left++;
        }
        
        return result;
    }
}