Generate a Pyramid Matrix

//int[][] result = new int[n][]; 生成了一个size为n的2D array 内容为null null null null....

public int[][] matrix(int n) {

if (n <= 0) return null;

int[][] result = new int[n][];

for (int i = 0; i < n; i++) {

result[i] = new int[i];

}

return result;

}