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

For example,
Given n = 3,

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

之前的一题是将螺旋结构的二维数组转成一维数组，此题正好相反，故只需将原来的位置上取下来的元素放回去。

public class SpiralMatrixII {
	public static void main(String[] args) {
		int[][] matrix = new SpiralMatrixII().generateMatrix(3);
		for(int[] r : matrix){
			for(int c : r)
				System.out.print(c + "\t");
			System.out.println();
		}
	}
	public int[][] generateMatrix(int n) {
		int[][] matrix = new int[n][n];
		if(n == 0)
			return matrix;
		int l = 0, r = matrix[0].length - 1;
		int u = 0, d = matrix.length - 1;
		int num = 1;
		while (l <= r && u <= d) {
			for (int i = l; i <= r; i++)
				matrix[u][i] = num++;
			u++;
			if (u > d)
				continue;
			for (int i = u; i <= d; i++)
				matrix[i][r] = num++;
			r--;
			if (l > r)
				continue;
			for (int i = r; i >= l; i--)
				matrix[d][i] = num++;
			d--;
			if (u > d)
				continue;
			for (int i = d; i >= u; i--)
				matrix[i][l] = num++;
			l++;
		}
		return matrix;
	}
}

LeetCode——Spiral Matrix II