NSString+test.h

- (void)depthFirstTraversal:(int)n {
    
    if (n > self.count-1) {
        NSLog(@"end of list");
        return;
    }
    
    if (n == 1) {
        NSLog(@"%@", self[n]);
        [self depthFirstTraversal:2 * n];
    }
    
    if (n % 2 == 0) {
        NSLog(@"%@", self[n]);
        [self depthFirstTraversal:2 * n];
    } else if (n % 2 == 1) {
        NSLog(@"%@", self[n]);
        [self depthFirstTraversal:2 * n + 1];
    }
    
}