59.Spiral Matrix II

题目链接：https://leetcode.com/problems/spiral-matrix-ii/description/

题目大意：构造蛇形矩阵。

法一：模板模拟。代码如下（耗时2ms）：

    public int[][] generateMatrix(int n) {
        int[][] res = new int[n][n];
        if(n == 0) {
            return res;
        }
        int i = 0, j = 0, num = 1;
        res[i][j] = num;
        while(num < n * n) {
            //从左往后
            while(j < n - 1 && res[i][j + 1] == 0) {
                res[i][++j] = ++num;
            }
            //从上到下
            while(i < n - 1 && res[i + 1][j] == 0) {
                res[++i][j] = ++num;
            }
            //从右到左
            while(j > 0 && res[i][j - 1] == 0) {
                res[i][--j] = ++num;
            }
            //从下到上
            while(i > 0 && res[i - 1][j] == 0) {
                res[--i][j] = ++num;
            }
        }
        return res;
    }