spiral Matrix II

螺旋矩阵

题目

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

Example:

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

解析重点

1.这可以看作一道数学找规律的题目，上，右，下，左是为循环一圈，坐标循环一圈时有规律的变化。上时横坐标不变，
纵坐标不断增加，到最右边终止上边的循环，横坐标+1.其他位置依此类推。
2.终止条件，中心时终止，即左<=右时终止。

java代码

class Solution {
    public static int[][] generateMatrix(int n) {
	    int[][] ret = new int[n][n];
	    int left = 0,top = 0;
	    int right = n -1,down = n - 1;
	    int count = 1;
	    while (left <= right) {
		    for (int j = left; j <= right; j ++) {
			    ret[top][j] = count++;
		    }
		    top ++;
		    for (int i = top; i <= down; i ++) {
			    ret[i][right] = count ++;
		    }
		    right --;
		    for (int j = right; j >= left; j --) {
			    ret[down][j] = count ++;
		    }
		    down --;
		    for (int i = down; i >= top; i --) {
			    ret[i][left] = count ++;
		    }
		    left ++;
	    }
	    return ret;
    }
}