Question 2 Solution

- (void)solution{
    
    NSArray *list = @[@"end", @"back", @"and", @"the", @"po", @"pu", @"lar", @"face"];  //populate with any list of strings here
    
    NSString *inputStr = @"facebackandthethethefacebackpopularlarpopu"; //or input any string here
    
    BOOL canBeFormed = [self checkForString:inputStr.mutableCopy inList:list];
    
    if (canBeFormed) {
        NSLog(@"YES! %@ can be formed from the list.", inputStr);
    }
    else{
        NSLog(@"NO! %@ cannot be formed from the list", inputStr);
    }
}

- (BOOL)checkForString:(NSMutableString *)inputStr inList:(NSArray *)list{
    
    for (NSString *str in list) {
        
        NSRange range = [inputStr rangeOfString:str options:NSCaseInsensitiveSearch];
        
        if (range.location != NSNotFound) {
            
            [inputStr deleteCharactersInRange:range];
            [self checkForString:inputStr inList:list];
        }
        
        if ([inputStr isEqualToString:@""]) {
            return YES;
        }
    }
    
    return NO;
}