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public class Solution {
    public int[][] generateMatrix(int n) {
        if (n < 1) return new int[0][0];
        int[][] res = new int[n][n];
        int left = 0, right = n - 1, up = 0, down = n - 1;
        int k = 1;
        while(true) {
            for (int i = left; i <= right; i++) {
                res[up][i] = k++;
            }
            if (++up > down) break;
            for (int i = up; i <= down; i++) {
                res[i][right] = k++;
            }
            if(--right < left) break;
            for (int i = right; i >= left; i--) {
                res[down][i] = k++;
            }
            if (--down < up) break;
            for (int i = down; i >= up; i--) {
                res[i][left] = k++;
            }
            if (++left > right) break;
        }
        return res;
    }
}