All unique
Input
s = "abcdef"
Output
True
No character repeats in the string.
Full lesson preview
Check if all characters in a string are unique
Determine whether a string contains all unique characters using frequency checking.
Python practice8 minStrings – Sliding Window & FrequencyBeginnerLast updated March 25, 2026
Problem statement
Given a string s, return True if all characters in s are unique (no repeats), otherwise return False. An empty string should be considered to have all unique characters (return True).
Task
Return True if every character in the input string appears only once; otherwise return False. Aim for O(n) time and O(1) extra space (character set dependent).
Examples
Input format
Function all_unique(s: str) -> bool
Output format
Return True if all characters are unique, otherwise False.
Constraints
0 <= len(s) <= 10^5. Use O(n) time.
Samples
Sample 1
Input
s = "aabc"
Output
False
'a' appears more than once.