给你一个正整数 n ，生成一个包含 1 到 n2 所有元素，且元素按顺时针顺序螺旋排列的 n x n 正方形矩阵 matrix 。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/spiral-matrix-ii
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class Solution {
    public int[][] generateMatrix(int n) {
        int[][] ret = new int[n][n];
        int idx = 1;
        int row1 = 0, col1 = 0;
        int row2 = n - 1, col2 = n - 1;

        while (row1 <= row2 && col1 <= col2) {
            if (row1 == row2) {
                for (int i = col1; i <= col2; ++i) {
                    ret[row1][i] = idx++;
                }
            } else if (col1 == col2) {
                for (int i = row1; i <= row2; ++i) {
                    ret[i][col1] = idx++;
                }
            } else {
                for (int i = col1; i < col2; ++i) {
                    ret[row1][i] = idx++;
                }
                for (int i = row1; i < row2; ++i) {
                    ret[i][col2] = idx++;
                }
                for (int i = col2; i > col1; --i) {
                    ret[row2][i] = idx++;
                }
                for (int i = row2; i > row1; --i) {
                    ret[i][col1] = idx++;
                }
            }
            row1++;
            col1++;
            row2--;
            col2--;
        }
        return ret;
    }
}