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Remove Duplicate Letters from a String

Problem Description​

Write a function that accepts a string as its input and returns the same string with duplicate letters removed.

Tip: If you can solve this challenge without a for-in loop, you can consider it “tricky” rather than “easy”.

Input​

  • A single string.

Output​

  • A string with duplicate letters removed, keeping only the first occurrence of each letter.

Constraints​

  • The function should preserve the order of the first occurrences of the letters.

Example​

Input:
"wombat"

Output:
"wombat"

Explanation:
The string "wombat" has no duplicate letters, so it remains unchanged.

Input:
"hello"

Output:
"helo"

Explanation:
The string "hello" has duplicate 'l' characters, so the second 'l' is removed.

Input:
"Mississippi"

Output:
"Misp"

Explanation:
The string "Mississippi" has multiple duplicate characters. After removing duplicates, the resulting string is "Misp".