Give a divide-and-conquer algorithm with a time complexity of (log ) to solve the problem of Finding the Single Letter

Finding the Single Letter: Given a string where all unique letters are supposed to appear twice one after one (e.g., aa). There may occur an error where one letter appears only once. Can you find the position of that single letter? Inputs with more than one single letter (e.g., bbaddc) or letters occurring more than twice (e.g., aaaa) are considered invalid. We assume all inputs are valid and you do not need to handle any exceptions.

Provide:

1. The pseudocode of your divide-and-conquer algorithm
2. The explanation of its complexity
3. A Python function find_single that implements your algorithm.