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