螺旋矩阵II

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 ]
]

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/spiral-matrix-ii
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建立top,bottom,left,right四个边界，创建n*n二维数组并逆时针赋值，当到达边界时，上、左边界自增，右、下边界自减，当top>bottom或left>right时，结束循环。代码如下：

    public int[][] generateMatrix(int n) {
        int[][] res = new int[n][n];
        int top = 0;
        int bottom = n-1;
        int left = 0;
        int right = n-1;
        int target = 1;
        //看了一下题解，有的大佬把判断循环是否结束的语句设置为target<=n*n
        while(top <= bottom || left <= right)
        {
            //从左往右
            for(int i = left; i <= right; i++)
            {
                res[top][i] = target++;
            }
            top++;
            //从上往下
            for(int i = top; i <= bottom; i++)
            {
                res[i][right] = target++;
            }
            right--;
            //从右往左
            for(int i = right; i >= left; i--)
            {
                res[bottom][i] = target++;
            }
            bottom--;
            //从下往上
            for(int i = bottom; i >= top; i--)
            {
                res[i][left] = target++;
            }
            left++;
            
        }
        return res;
    }