访问矩阵元素的索引

矩阵：
[1, 2, 3, 4]
[5, 6, 7, 8]
[9, 10, 11, 12]

索引示意图：
(0, 0) (0, 1) (0, 2) (0, 3)
(1, 0) (1, 1) (1, 2) (1, 3)
(2, 0) (2, 1) (2, 2) (2, 3)

matrix = [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]]
element = matrix[1][1]
print(element)  # 输出 6

int[][] matrix = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}};
int element = matrix[1][1];
System.out.println(element);  // 输出 6

int matrix[3][4] = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}};
int element = matrix[1][1];
cout << element << endl;  // 输出 6