求数组的 交集，并集，差集
NSArray *array1 = @[@"1",@"2",@"3"];
NSArray *array2 = @[@"1",@"5",@"6"];
NSMutableSet *set1 = [NSMutableSet setWithArray:array1];
NSMutableSet *set2 = [NSMutableSet setWithArray:array2];
[set1 unionSet:set2]; //取并集后 set1中为1，2，3，5，6
[set1 intersectSet:set2]; //取交集后 set1中为1
[set1 minusSet:set2]; //取差集后 set1中为2，3，5，6
判断一个数组是否为另一个数组的子集
NSArray *array3 = @[@"1",@"2"];
NSArray *array4 = @[@"1",@"2",@"6"];
NSSet *set3 = [NSSet setWithArray:array3];
NSSet *set4 = [NSSet setWithArray:array4];
BOOL isSub = [set3 isSubsetOfSet:set4]; //isSub为YES
判断某个对象是否在某个数组中
BOOL isExist = [array3 containsObject:@"1"]; //isExist为YES
