添加元素
NSMutableArray *myColors;
myColors = [NSMutableArray arrayWithObjects: @"Red", @"Green", @"Blue", @"Yellow", nil];
[myColors addObject: @"Indigo"];
[myColors addObject: @"Violet"];
//Add objects from an NSArray
NSArray *myArray = @[@"Purple",@"Orange"];
[myColors addObjectsFromArray:myArray];