Spiral Matrix II

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 ]
]

Solution: 按照上右下左的顺序放，这道题是直接给出1到n^2， 每个元素只访问一次，时间复杂度是O(n^2), space complexity is O(n^2)。

public class Solution {
    public int[][] generateMatrix(int n) {
        int[][] result = new int[n][n];
        
        if(n == 0)
            return result;
        
        int top = 0, bottom = n - 1;
        int left = 0, right = n - 1;
        int k = 1;
        
        while(top < bottom && left < right) {
            for(int i = left; i < right; i++) {
                result[top][i] = k++;
            }
            for(int i = top; i < bottom; i++) {
                result[i][right] = k++;
            }
            for(int i = right; i > left; i--) {
                result[bottom][i] = k++;
            }
            for(int i = bottom; i > top; i--) {
                result[i][left] = k++;
            }
            top++;
            bottom--;
            left++;
            right--;
        }
        
        if(n % 2 != 0) {    //如果是奇数行，中心点要单独加
            result[n / 2][n / 2] = k;
        }
        
        return result;
    }
}