Is First Letter to Appear Twice easy or hard?

First Letter to Appear Twice is classified as an Easy problem. It tests basic understanding of hash sets, arrays, and string traversal. The optimal solution requires a single pass through the string with constant-time lookups.

First Letter to Appear Twice Python/Java solution

Python solutions usually use a set to track seen characters during iteration. Java implementations often use either a HashSet or a 26-length integer array for counting occurrences. Both implementations achieve O(n) time and O(1) space complexity.

How to solve First Letter to Appear Twice in O(n)?

Iterate through the string while storing characters in a hash set. For every character, check if it already exists in the set. If it does, return it immediately because this is the earliest second occurrence. Otherwise insert it into the set and continue scanning.

What is the best approach for First Letter to Appear Twice?

The most common approach is scanning the string once while storing previously seen characters in a hash set. If the current character already exists in the set, it is the first letter whose second occurrence appears earliest. This runs in O(n) time with O(1) space because the set size is limited to 26 lowercase letters.

Is First Letter to Appear Twice asked at Google/Amazon/Meta?

This problem represents a common interview pattern involving hash tables and duplicate detection in strings. Variations of this pattern appear in interviews at companies like Amazon and Google where candidates must detect repeated elements efficiently using O(n) time solutions.

What data structure is used in First Letter to Appear Twice?

Typical solutions use a hash set to track characters that have already appeared. Another common structure is a fixed-size array of length 26 that stores frequency counts for lowercase letters. Both allow constant-time lookups while scanning the string.

What is the time complexity of First Letter to Appear Twice?

The optimal solutions run in O(n) time where n is the length of the string. Each character is processed once with constant-time lookup using either a hash set or a fixed-size counting array. Space complexity is O(1) due to the limited alphabet size.

Description Hints 2 Solution Video 3:18

2351. First Letter to Appear Twice

Easy74.9% acceptance2 companies

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.
s will contain at least one letter that appears twice.

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.

Asked at 2 companies

Google Apple

Topics

Hints open one at a time

Hint 1

Iterate through the string from left to right. Keep track of the elements you have already seen in a set.

Hint 2

If the current element is already in the set, return that element.

3:18

Still stuck?Read 4 approaches, or watch a walkthrough.

Open solution