Example - s = 'a1b'
Input
a1b
Output
['a1b', 'a1B', 'A1b', 'A1B']
For 'a' try 'a' then 'A'; for 'b' try 'b' then 'B'. Digits unchanged.
Full lesson preview
Generate letter case permutations
Return all permutations of a string where alphabetic characters can be either lowercase or uppercase.
Python practice15 minRecursion & BacktrackingIntermediateLast updated March 27, 2026
Problem statement
Given a string s consisting of letters and digits, generate all possible strings by changing the case of each letter independently. Digits should remain the same. Return the list of permutations in a deterministic order: process characters left-to-right, and for letters try lowercase first then uppercase.
Task
Use recursion/backtracking to toggle case on letters and keep digits unchanged to produce all permutations.
Examples
Input format
A single string s (letters and digits).
Output format
A list of strings representing all case permutations (deterministic order).
Constraints
0 <= len(s) <= 12. Maintain input character order; for each letter try lowercase before uppercase.
Samples
Sample 1
Input
3z4
Output
['3z4', '3Z4']
Only the letter 'z' has case variations.