59. 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 ]
]

public class Solution {
    public int[][] generateMatrix(int n) {
        int[][] a = new int[n][n];
        if(n == 0) return a;
        if(n == 1) {
            a[0][0] = 1;
            return a;
        }
        int top = 0;
        int bottom = n -1;
        int left = 0;
        int right = n - 1;
        int num = 1;
        while(top <= bottom && left <= right){
            //right to left in top
            for(int i = left ; i <= right ; i++){
                a[top][i] = num;
                num++;
            }
            top++;
            
            //top to bottom in right
            for(int i = top ; i <= bottom ; i++){
                a[i][right] = num;
                num++;
            }
            right--;
           
            // right ->  left in bottom 
            if(bottom >= top){
                for(int i = right ; i >= left ; i--){
                    a[bottom][i] = num;
                    num++;
                }
                bottom--;
            }
            
            //bottom -> top in left
            if(right >= left){
                for(int i = bottom ; i >= top ; i--){
                    a[i][left] = num;
                    num++;
                }
                left++;
            }
        }
        return a;
    }
}