public class Solution {
    public int[][] generateMatrix(int n) {
        if(n<0)
        {
            return null;
        }
        int [][]matrix=new int[n][n];
        int up=0;
        int down=n-1;
        int left=0;
        int right=n-1;
        int row=0;
        int col=0;
        int currentValue=1;
        while(true)
        {
            //向右
            for(col=left;col<=right;col++)
            {
                matrix[row][col]=currentValue++;
            }
            up++;
            if(up>down)
            {
                break;
            }
            col--;
            //向下
            for(row=up;row<=down;row++)
            {
                matrix[row][col]=currentValue++;
            }
            right--;
            if(left>right)
            {
                break;
            }
            row--;
            //向左
            for(col=right;col>=left;col--)
            {
                matrix[row][col]=currentValue++;
            }
            down--;
            if(up>down)
            {
                break;
            }
            col++;
            //向上
            for(row=down;row>=up;row--)
            {
                matrix[row][col]=currentValue++;
            }
            left++;
            if(left>right)
            {
                break;
            }
            row++;
        }
        return matrix;
    }
}