How do you solve First Letter to Appear Twice on LeetCode?

First Letter to Appear Twice (LeetCode #2351) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(n) space complexity. Key insight: Using a set allows for O(1) average time complexity for lookups.

What is the brute force solution for First Letter to Appear Twice?

The brute force approach for First Letter to Appear Twice is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute force approach checks every character against all previous characters to see if it has appeared before. This is straightforward but inefficient for larger strings. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve First Letter to Appear Twice?

First Letter to Appear Twice uses the following concepts: Hash Table, String, Bit Manipulation, Counting. The recommended patterns to study are: Hash Map, Array.

What are the interview tips for First Letter to Appear Twice?

Clearly explain your thought process and why you choose a specific approach. Consider edge cases, such as strings with only two characters. Practice writing clean and efficient code, as readability is important.

What are common mistakes when solving First Letter to Appear Twice?

Not using a data structure to track seen characters, leading to inefficient solutions. Confusing the order of appearances when multiple characters could be duplicates.

#2351

First Letter to Appear Twice

Easy

Hash TableStringBit ManipulationCountingHash MapArray

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(n)

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Intuition

Time O(n)Space O(n)

Using a set to track seen characters allows us to check for duplicates in constant time, making the solution efficient as we only traverse the string once.

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Algorithm

4 steps

1Step 1: Initialize an empty set to keep track of seen characters.
2Step 2: Iterate through each character in the string.
3Step 3: For each character, check if it is in the set. If it is, return that character.
4Step 4: If not, add the character to the set.

solution.py6 lines

1def firstLetterToAppearTwice(s):
2    seen = set()
3    for char in s:
4        if char in seen:
5            return char
6        seen.add(char)

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Complexity note: The time complexity is O(n) because we traverse the string once. The space complexity is O(n) due to the set storing characters we've seen.

1Using a set allows for O(1) average time complexity for lookups.
2The order of characters matters; we need to return the first duplicate.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.