First Letter to Appear Twice — LeetCode 2351 Python Solution

The problem

Given a string s consisting of lowercase English letters, return the first letter to appear twice. Note: A letter a appears twice before another letter b if the second occurrence of a is before the second occurrence of b.

Example

Input

s = "abccbaacz"

Output

"c"

Explanation

The letter 'a' appears on the indexes 0, 5 and 6.

Python solution

class Solution:
    def repeatedCharacter(self, s: str) -> str:
        cnt = Counter()
        for c in s:
            cnt[c] += 1
            if cnt[c] == 2:
                return c

Complexity

Pattern: Bit Manipulation

Use XOR, masks and the low-bit trick to replace whole data structures with an integer. LeetCode 2351. First Letter to Appear Twice is filed here because LeetCode tags it Bit Manipulation, which is the vocabulary this hub collects.

The bit manipulation guide has the Python template for the pattern and the 194 LeetCode problems that use it.

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