59. Spiral Matrix II

class Solution {
    public int[][] generateMatrix(int n) {
        int[][] matrix=new int[n][n];
        generateMatrix(0, 0, 0, n-1, n-1, matrix);
        return matrix;
    }
    private void generateMatrix(int num, int top, int left, int bottom, int right, int[][] matrix){
        if(top>bottom||left>right)
            return;
        for(int k=left;k<=right;k++)
            matrix[top][k]=++num;
        for(int k=top+1;k<=bottom;k++)
            matrix[k][right]=++num;
        if(top!=bottom)
            for(int k=right-1;k>=left;k--)
                matrix[bottom][k]=++num;
        if(left!=right)
            for(int k=bottom-1;k>top;k--)
                matrix[k][left]=++num;
        generateMatrix(num, top+1, left+1, bottom-1, right-1, matrix);
    }
}