59. 螺旋矩阵 II (Java) Leecode

解题思路：

与 54.螺旋矩阵相似
只是换成了输入n，生成矩阵。

留意题目要求输出从1开始，所有num= 1，后续 < n * n

class Solution {
    public int[][] generateMatrix(int n) {
    int[][]matrix = new int[n][n];
    
    int upper = 0, left = 0, lower = n - 1, right = n - 1;

    int num = 1;

    while(num <= n * n){
        //从左到右
        if(upper <= lower){
            for(int i = left; i <= right; i++){
                matrix[upper][i] = num++;
            }

            upper++;//上边界下移
        }
        //从上到下
        if(left <= right){
            for(int j = upper; j <= lower; j++){
                matrix[j][right] = num++;
            }
            right--;//右边界左移
        }

        //从右到左
        if(upper <= lower){
            for (int i = right; i >= left; i--){
                matrix[lower][i] = num++;
            }

            lower--;//下边界上移
        }
        //从下到上
        if(left <= right){
            for(int j = lower; j >= upper; j--){
                matrix[j][left] = num++;
            }

            left++;//左边界右移
        }
    }
    return matrix;
    }  
}