Short Python solution
\nh, result = defaultdict(set), ""\nfor c in s:\n\tu = c.upper()\n\th[u].add(c)\n\tif len(h[u]) == 2:\n\t\tresult = max(u, result)\nreturn result\n
```\nh, result = defaultdict(set), ""\nfor c in s:\n\tu = c.upper()\n\th[u].add(c)\n\tif len(h[u]) == 2:\n\t\tresult = max(u, result)\nreturn result\n```
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