First Letter to Appear Twice
Given a string s consisting of lowercase English letters, return the first letter to appear twice.
Given a string s consisting of lowercase English letters, return the first letter to appear twice.
Note:
Example 1
Input: s = "abccbaacz"
Output: "c"
Explanation: The letter 'a' appears on the indexes 0, 5 and 6. The letter 'b' appears on the indexes 1 and 4. The letter 'c' appears on the indexes 2, 3 and 7. The letter 'z' appears on the index 8. The letter 'c' is the first letter to appear twice, because out of all the letters the index of its second occurrence is the smallest.
Example 2
Input: s = "abcdd"
Output: "d"
Explanation: The only letter that appears twice is 'd' so we return 'd'.
Constraints
- 2 <= s.length <= 100
- s consists of lowercase English letters.
- s has at least one repeated letter.
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