1. How would you copy a custom object like a "Node" from a search tree in Objective C?

Version 1

@interface Node : NSObject <NSCopying>

@end

@implementation Node
- (id) copyWithZone: (NSZone*)zone {
    id copy = [[[self class] alloc] init];

    if (copy) {
        [copy setVal: self.val];
        [copy setRootNode: [[self.rootNode copyWithZone:zone]]];
        [copy setLeftNode: [[self.leftNode copyWithZone:zone]]];
        [copy setRightNode: [[self.rightNode copyWithZone:zone]]];
    }
    return copy;
}    
@end


//Now you can do something like:
Node* myNode = [[Node alloc] init];
Node* newNode = [myNode copy];

Version 2

https://gist.github.com/zats/c075815e1fa24b001fb2

// Node.h
#import <Foundation/Foundation.h>

@interface Node : NSObject <NSCopying, NSMutableCopying>
@property (nonatomic, weak, readonly) Node *parent;
@property (nonatomic, strong, readonly) Node *left;
@property (nonatomic, strong, readonly) Node *right;
- (instancetype)initWithParent:(Node *)parent left:(Node *)left right:(Node *)right;
@end

@interface MutableNode : Node
@property (nonatomic, weak) Node *parent;
@property (nonatomic, strong) Node *left;
@property (nonatomic, strong) Node *right;
@end

// Node.m
#import "Node.h"

@interface Node ()
@property (nonatomic, weak) Node *parent;
@property (nonatomic, strong) Node *left;
@property (nonatomic, strong) Node *right;
@end

@implementation Node
- (instancetype)initWithParent:(Node *)parent left:(Node *)left right:(Node *)right {
    self = [super init];
    if (!self) {
        return nil;
    }
    self.parent = parent;
    self.left = left;
    self.right = right;
    return self;
}
- (id)copyWithZone:(NSZone *)zone {
    return self;
}
- (id)mutableCopyWithZone:(NSZone *)zone {
    MutableNode *node = [[MutableNode allocWithZone:zone] init];
    node.parent = self.parent;
    node.left = self.left;
    node.right = self.right;
    return node;
}
- (void)setLeft:(Node *)left {
    _left = left;
    if (left.parent != self) {
        left.parent = self;
    }
}
- (void)setRight:(Node *)right {
    _right = right;
    if (right.parent != self) {
        right.parent = self;
    }
}
- (NSString *)description {
    NSMutableString *string = [NSMutableString stringWithFormat:@"<%@:%p>", [self class], self];
    if (!self.parent && !self.left && !self.right) {
        return [string copy];
    }
    [string appendString:@" {"];
    if (self.parent) {
        [string appendFormat:@" parent = <%@:%p>", [self.parent class], self.parent];
    }
    if (self.left) {
        [string appendFormat:@" left = <%@:%p>", [self.left class], self.left];
    }
    if (self.right) {
        [string appendFormat:@" right = <%@:%p>", [self.right class], self.right];
    }
    [string appendString:@" }"];
    return [string copy];
}
@end

@implementation MutableNode
- (id)copyWithZone:(NSZone *)zone {
    Node *node = [[Node allocWithZone:zone] init];
    [node setValue:self.parent forKey:@"parent"];
    [node setValue:self.left forKey:@"left"];
    [node setValue:self.right forKey:@"right"];
    return node;
}
@end

2. Inorder Enum Binary Tree

+ (void) inorderTraversal:(TreeNode*) root block:(void(^)(NSString*))block {
    [self inorderTraversal:root.left block:block];
    block(root.val);
    [self inorderTraversal:root.right block:block];
}

// use 
NSMutableArray* res = [NSMutableArray array];
int (^readingBlock)(NSString*) = ^(NSString* cur) {
    [res addObject:cur];
};

+ (void) inorderTraversal:(TreeNode*) root array:(NSMutableArray*)array {
    [self inorderTraversal:root.left block:block];
    [array addObject:root.val];
    [self inorderTraversal:root.right block:block];
}