思路：与上一篇类似。仅仅要不越界就ok

public class Solution {

	public int[][] generateMatrix(int n) {

	    if (n == 0)

			return new int[0][0];

		int[][] matrix = new int[n][n];

		int x1 = 0;

		int y1 = 0;

		int x2 = matrix.length - 1;

		int y2 = matrix[0].length - 1;

		int i = 0, j = 1;

		while (x1 <= x2 && y1 <= y2) {

			// up row

			for (i = y1; i <= y2; ++i, j++)

				matrix[x1][i] = j;

			// right column

			for (i = x1 + 1; i <= x2; ++i, j++)

				matrix[i][y2] = j;

			// bottom row

			for (i = y2 - 1; x2 != x1 && i >= y1; --i, j++)

				matrix[x2][i] = j;

			// left column

			for (i = x2 - 1; y1 != y2 && i > x1; --i, j++)

				matrix[i][y1] = j;

			x1++;

			y1++;

			x2--;

			y2--;

		}

		return matrix;

	}

}

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