Spiral Matrix

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 ]
]

public class Solution {
    public int[][] generateMatrix(int n) {
        int startX = 0;
        int startY = 0;
        int endX = n - 1;
        int endY = n - 1;
        
        int[][] matrix = new int[n][n];
        int startValue = 1;
        
        while (startX <= endX) {
            startValue = fillNumber(startX, startY, endX, endY, matrix, startValue);
            startX++;
            startY++;
            endX--;
            endY--;
        }
        return matrix;
    }
    
    public int fillNumber(int startX, int startY, int endX, int endY, int[][] matrix, int startV) {
        if (startX == endX) {
            matrix[startX][startY] = startV;
            return -1;
        }
        
        for (int i = startY; i <= endY; i++) {
            matrix[startX][i] = startV;
            startV++;
        }
        for (int i = startX + 1; i <= endX; i++) {
            matrix[i][endY] = startV;
            startV++;
        }
        
        for (int i = endY - 1; i >= startY; i--) {
             matrix[endX][i] = startV;
            startV++;
        }
        
        for (int i = endX - 1; i >= startX + 1; i--) {
             matrix[i][startY] = startV;
             startV++;
        }
        
        return startV;
    }
}