Basic compression
Input
aaabbc
Output
a3b2c1
Three 'a's become 'a3', two 'b's become 'b2', one 'c' becomes 'c1'.
Full lesson preview
Compress repeated characters using counts
Implement run-length encoding by collapsing consecutive repeated characters into character+count pairs.
Python practice22 minString PatternsAdvancedLast updated March 18, 2026
Problem statement
Given a string s, return a compressed version using run-length encoding: each consecutive group of the same character is replaced by the character followed by the number of times it appears consecutively. The function should be case-sensitive and preserve non-letter characters. If the input is empty, return an empty string.
Task
Write a function that compresses a string by replacing runs of identical characters with the character followed by the count of repetitions (always include the count, even when it is 1).
Examples
Input format
A single string s.
Output format
A single string representing the run-length encoded form of s.
Constraints
0 <= len(s) <= 10^6. Characters may include letters, digits, punctuation, and whitespace. Must be O(n) time and O(1) additional space (excluding output).
Samples
Sample 1
Input
WWWWaa!!
Output
W4a2!2
Four 'W's, two 'a's, two '!' characters.