[leetcode]59. Spiral Matrix II螺旋遍历矩阵2

Given a positive integer n, generate a square matrix filled with elements from 1 to n^2 in spiral order.

Example:

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

题意：

给定n， 把1至n ^ 2所有的数，按照螺旋顺序填入方阵。

code

 /*
 Time: O(n^2)
 Space: O(n^2)
 */
 class Solution {
     public int[][] generateMatrix(int n) {
         int[][] matrix = new int[n][n];
         if (n == 0) return matrix;
         int beginX = 0, endX = n - 1;
         int beginY = 0, endY = n - 1;
         int num = 1;
         while (true) {
             for (int j = beginX; j <= endX; ++j) matrix[beginY][j] = num++;
             if (++beginY > endY) break;

             for (int i = beginY; i <= endY; ++i) matrix[i][endX] = num++;
             if (beginX > --endX) break;

             for (int j = endX; j >= beginX; --j) matrix[endY][j] = num++;
             if (beginY > --endY) break;

             for (int i = endY; i >= beginY; --i) matrix[i][beginX] = num++;
             if (++beginX > endX) break;
         }
         return matrix;
     }
 }