Inspired by the common binary search algorithm, this design would be a binary or tertiary search on the next desired letter. The selection tree would appear under a list of the already selected letters. After a new letter is selected, the tree resets to the root node eg.
Below are two potential visualizations of this, the tree representation and the list representation, where a user would use the left or right arrow keys to narrow in on the desired letter. Both diagrams show the same progression of selecting the next letter to be an “N”.
Binary Search Letter Selection Tree
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