如何在Objective-C中声明字符串类型的二维数组？

NSArray<NSArray<NSString *> *> *twoDimensionalArray = @[
    @[@"string1", @"string2", @"string3"],
    @[@"string4", @"string5", @"string6"],
    @[@"string7", @"string8", @"string9"]
];

NSMutableArray<NSMutableArray<NSString *> *> *mutableTwoDimensionalArray = [NSMutableArray new];

[mutableTwoDimensionalArray addObject:@[@"string1", @"string2", @"string3"]];
[mutableTwoDimensionalArray addObject:@[@"string4", @"string5", @"string6"]];
[mutableTwoDimensionalArray addObject:@[@"string7", @"string8", @"string9"]];