Shortest Distinct Substring (N=3 variant)

Problem: Shortest Distinct Substring (N=3 variant) You are given a lowercase string s and an integer N = 3. A substring is considered distinct if all characters inside it are pairwise different. Find the shortest distinct substring in s. If multiple substrings have the same shortest length, return the leftmost one. If no distinct substring exists, return an empty string. Note: Since N=3 is given, the answer length will be at most 3. If a length-1 or length-2 substring is already distinct, you should return the shorter one. Input (stdin) One line: string s Output (stdout) One string: the shortest distinct substring (or empty string) Constraints 1 <= len(s) <= 2 * 10^5 s consists of a-z Examples / Tests Test 1 aab Output ab Test 2 aaa Output Test 3 abc Output a Test 4 abba Output ab Test 5 zzxyzz Output z Example Input aab Output ""