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    <p>In order to get the elements of maximum depth, you will need to keep track of two things simultaneously for every subtree visited by <code>btree_reduce</code>: The maximum depth of that subtree, and the elements found at that depth. Wrap this information up in some data structure, and you have your type <code>'b</code> (according to <code>btree_reduce</code>'s signature).</p> <p>Now, when you need to combine two subtree results in the function you provide to <code>btree_reduce</code>, you have three possible cases: "Left" sub-result is "deeper", "less deep", or "of equal depth" to the "right" sub-result. Remember that the sub-result represent the depths and node values of the deepest nodes in each subtree, and think about how to combine them to gain the depth and the values of the deepest nodes for the current tree.</p> <p>If you need more pointers, I have an implementation of <code>btree_deepest</code> ready which I'm just itching to share; I've not posted it yet since you specifically (and honorably) asked for hints, not the solution.</p>
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    1. POFind the deepest element of a Binary Tree in SML
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    1. COThanks! I am still a little confused though. I understand that because SML is functional it will resolve everything to the base case before it starts to run the function `'f` and that my return type `'b` is going to be something like `(r1: list, r2: int)` with r1 being the list of elements and r2 the height. I am still not sure how to specifically compare the "left" r2 to the "right" r2 and then subsequently pass the "correct" r1 up. (Which I guess really is the meat of the issue still...) Thanks again for your help!
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    2. COLet's say your function `f` is being called with the argument `((left_elements, left_depth), x, (right_elements, right_depth))`. Now, if `left_depth > right_depth`, you keep your `left_elements` for your new result, and increase depth by 1. Vice versa if `left_depth < right_depth`. In case `left_depth = right_depth`, combine your `left_elements` and `right_elements` lists, since the exercise asked to return all elements of the maximum depth.
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    3. COAh, I feel so dumb right now... I've updated my solution attempt above but it is now giving me an error on `if left_dep > right_dep then (left_ele, left_dep+1) else (right_ele, right_dep+1)` saying that types of if branches do not agree. I would just declare a type with left_elements:int list for example but it has to handle both integers and strings interchangeably. Am I at least on the right path?
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