Spiral Matrix II:螺旋打印方阵

Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

For example,
Given n = 3,

You should return the following matrix:

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

思路：和上一题Spiral Matrix类似，只不过变成方阵了，数据范围从1到n^2。

class Solution {
    public int[][] generateMatrix(int n) {
        int[][] x = new int[n][n];
        int digit = 1;
        int rowstart = 0;
        int colstart = 0;
        int rowend = n - 1;
        int colend = n - 1;
        while(colend>=colstart&&rowstart<=rowend){
            for(int i = colstart ;i<=colend ; i++){
                x[rowstart][i] = digit++;
            }
            rowstart++;
            for(int i = rowstart ;i<=rowend ; i++){
                x[i][colend] = digit++;
            }
            colend--;
            if(colend>=colstart){
                for(int i = colend ;i>=colstart ; i--){
                    x[rowend][i] = digit++;
                }
            }            
            rowend--;
            if(rowstart<=rowend){
               for(int i = rowend ;i>=rowstart ; i--){
                    x[i][colstart] = digit++;
                } 
            }
            colstart++;            
        }
        return x;
    }
}