How do I traverse two identical trees and return a target value?

Question

Can someone tell me why the console.log outputs the right answer, however, the return outputs null?

I am trying to traverse two identical trees, and then when a target value is found in a node in the original tree, I am trying to return the same node value in the cloned tree.

I really appreciate any help you can provide.

Original problem: https://leetcode.com/problems/find-a-corresponding-node-of-a-binary-tree-in-a-clone-of-that-tree/

Codebase:

var getTargetCopy = function(original, cloned, target) {
    if(original === null) {
        return null
    }
    
    original.left = getTargetCopy(original.left, cloned.left, target)
    original.right = getTargetCopy(original.right, cloned.right, target)
    
    if(original.val === target.val) {
        console.log(cloned.val)
        return cloned.val
    }
};

Inputs:

Your input:
original = [7,4,3,null,null,6,19]
cloned = [7,4,3,null,null,6,19]
target.val = 3

Console.log:

stdout:
3

Outputs:

Output:
null

Desired answer:

Expected:
3

Answer 1

Some issues:

1. The function is supposed to return a node, not a value, so return cloned.val is wrong. It should be return cloned

2. With the following statements you are mutating the original tree:

    original.left = getTargetCopy(original.left, cloned.left, target) original.right = getTargetCopy(original.right, cloned.right, target)

   That should not happen.

3. with if(original.val === target.val) your code could have a false positive when target.val is not a unique value. Instead you should compare the node reference .

4. When you have a match, you should not have to recurse deeper, so perform this test before doing the recursive calls.

5. When a recursive call returns a node, you should immediately return that node and not do any other recursive call.

Here is your code corrected:

var getTargetCopy = function(original, cloned, target) {
    if(original === null) {
        return null
    }
    
    if(original === target) {
        return cloned;
    }

    return getTargetCopy(original.left, cloned.left, target)
            || getTargetCopy(original.right, cloned.right, target)
    
};

Making it a bit more condensed, it can be:

var getTargetCopy = (original, cloned, target) =>
    !original || original === target 
        ? cloned
        : getTargetCopy(original.left, cloned.left, target)
          || getTargetCopy(original.right, cloned.right, target);