lintcode-medium-Spiral Matrix II

public class Solution {
    /**
     * @param n an integer
     * @return a square matrix
     */
    public int[][] generateMatrix(int n) {
        // Write your code here
        
        if(n == 0)
            return new int[0][0];
        
        int[][] res = new int[n][n];
        int[] num = new int[1];
        num[0] = 1;
        
        for(int i = 0; i < n / 2; i++)
            helper(res, i, num);
        
        if(n % 2 == 1) 
            res[n / 2][n / 2] = num[0];
        
        return res;
    }
    
    public void helper(int[][] res, int layer, int[] num){
        int n = res.length;
        
        int start = layer;
        int end = n - layer - 1;
        
        if(start == end){
            res[start][start] = num[0];
            return;
        }
        
        for(int i = 0; i < end - start; i++){
            res[start][start + i] = num[0];
            num[0] = num[0] + 1;
        }
        for(int i = 0; i < end - start; i++){
            res[start + i][end] = num[0];
            num[0] = num[0] + 1;
        }
        for(int i = 0; i < end - start; i++){
            res[end][end - i] = num[0];
            num[0] = num[0] + 1;
        }
        for(int i = 0; i < end - start; i++){
            res[end - i][start] = num[0];
            num[0] = num[0] + 1;
        }
        
        return;
    }
    
}