向数组指定位置插入元素

NSMutableArray *array = [[NSMutableArray alloc] init];
for(int i = 0;i<5;i++){
[array addObject:[NSNull null]];
}
初始化一个null的数组

NSArray *a = [[NSArray alloc] initWithObjects:@"1",@"2",@"3",nil];
[array replaceObjectAtIndex:2 withObject:a];
NSLog(@"array: %@",array);
[a release];
[array release];