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LEETFLIX

First Letter to Appear Twice

EasyHashingStringBitmask
Solve on LeetCode

Easy · hashing solution for First Letter to Appear Twice. Python, Go, and a step-by-step animation.

01

Problem

Read the prompt, flip through the examples, then steal the idea.

LeetCode 2351. First Letter to Appear Twice. The copyrighted prompt is on LeetCode. This page is a working hashing solution for this problem: editable input, Python, Go, and a step-by-step animation.

Constraints

  • —Player inputs are capped so the animation stays readable.
  • —Match LeetCode's own constraints when you submit there.

Examples

Input

nums =
[1, 2, 3, 1]

Output

true

The hash set fires when a value is seen twice.

02

Solution

Python and Go. The walkthrough below runs this same code.

Approach 1Optimal▶ Animated

First Letter to Appear Twice — hashing

Remember every value you have already seen. A hit means the pattern fired. Built for LeetCode 2351. First Letter to Appear Twice.

A hash set answers “have I seen this before?” in expected O(1). Walk once, look up, then insert — that order is what keeps an element from pairing with itself.

Algorithm

  1. Create an empty set seen.
  2. For each value x, if x is already in seen, return true.
  3. Otherwise insert x and continue.
  4. If the walk finishes, return false.
Time
O(n)
Space
O(n)
Solution.py
class Solution:    def firstLetterToAppearTwice(self, nums: list[int]) -> bool:        seen: set[int] = set()        for x in nums:            if x in seen:                return True            seen.add(x)        return False
Watch this run

03

Interactive walkthrough

Playing “First Letter to Appear Twice — hashing”. Edit the input, hit Run, then scrub.

Approach: First Letter to Appear Twice — hashing

Loading animation…