leetcode : Spiral Matrix II

Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

For example,

Given n = 3,

You should return the following matrix:

[

 [ 1, 2, 3 ],

 [ 8, 9, 4 ],

 [ 7, 6, 5 ]

]

 tag: 坐标变换。  递归。 矩阵

 

public class Solution {
    public int[][] generateMatrix(int n) {
        
        if(n <= 0) {
            return new int[0][0];
        }
        
        if(n == 1) {
            int[][] matrix = new int[1][1];
            matrix[0][0] = 1;
            return matrix;
        }
        
        int x = 0;
        int y = 0;
        int rows = n;
        int cols = n;
        int[][] matrix = new int[n][n];
        int count = 1;
        helper(matrix, x, y, rows, cols, count);
        return matrix;
    }
    
    public void helper(int[][] matrix, int x, int y, int rows, int cols, int count) {
        
        if(rows <= 0 || cols <= 0) {
            return;
        }
        
        // first line
        for(int i = 0; i < cols; i++) {
            matrix[x][y + i] = count++;
        }
        
        // right line
        for(int i = 1; i < rows - 1; i++) {
            matrix[x + i][y + cols - 1] = count++;
        }
        
        // last line
        if(rows > 1) {
            for(int i = cols - 1; i >= 0; i-- ) {
                matrix[x + rows - 1 ][y + i] = count++;
            }
        }
        
        if(cols > 1) {
            for(int i = cols - 2; i > 0; i--) {
                matrix[x + i][y] = count++;
            }
        }
        
        helper(matrix, x + 1, y + 1, rows - 2, cols - 2,count);
    }
}