Collapse runs of identical letters
Input
'aaabcc' -> remove_adjacent_duplicates('aaabcc')
Output
'abc'
Runs 'aaa' -> 'a', 'cc' -> 'c', so the result is 'abc'.
Full lesson preview
Remove adjacent duplicate characters
Collapse consecutive identical characters in a string, keeping only the first of each adjacent run.
Python practice15 minString PatternsIntermediateLast updated March 18, 2026
Problem statement
Write a function that takes a string and removes adjacent duplicate characters: whenever one or more identical characters appear consecutively, they should be collapsed into a single instance of that character. The relative order of non-adjacent characters must be preserved. The function should be case-sensitive (i.e., 'A' and 'a' are different).
Task
Practice linear scanning of strings and understanding run-length-like transformations.
Examples
Input format
A single string passed to remove_adjacent_duplicates(s).
Output format
A string with adjacent duplicates removed.
Constraints
Must run in O(n) time and O(1) extra space (ignoring result storage). Handle empty strings and preserve case-sensitivity.
Samples
Sample 1
Input
aaabbc
Output
abc
aaa -> a, bb -> b, c -> c