Is Letter Case Permutation easy or hard?

The problem is rated Medium because it requires recognizing a permutation pattern. The logic is straightforward once you see that each letter introduces two choices, but generating all combinations efficiently using recursion or bit masks can be tricky for beginners.

How to solve Letter Case Permutation in O(n)?

Generating all permutations cannot be done in O(n) because the output size itself is exponential. If the string has k letters, there are 2^k valid outputs. The optimal solutions—backtracking or bitmask enumeration—run in O(2^k * n), which is proportional to the number of results produced.

What is the best approach for Letter Case Permutation?

Backtracking is the most common approach. It explores two branches for every letter (lowercase and uppercase) while keeping digits unchanged. The algorithm generates 2^k permutations where k is the number of alphabetic characters, resulting in O(2^k * n) time complexity.

What data structure is used in Letter Case Permutation?

The solution typically uses recursion with a dynamic string or character array to build permutations. A list or array stores the final results. Some implementations also track letter positions and use bitmasks to represent case choices.

What is the time complexity of Letter Case Permutation?

The time complexity is O(2^k * n), where k is the number of letters in the string and n is the string length. Each letter doubles the number of possible permutations, and constructing each resulting string requires up to n operations.

Letter Case Permutation Python or Java solution approach?

Both Python and Java implementations usually use backtracking. A recursive function processes one character at a time, branching into lowercase and uppercase when encountering a letter. Iterative bitmask solutions are also common in Python and JavaScript due to simple bit operations.

Is Letter Case Permutation asked at Google, Amazon, or Meta?

Variants of this problem appear in interviews at companies like Amazon, Google, and Microsoft. It tests understanding of recursion, permutation generation, and bit manipulation patterns that commonly appear in string and backtracking interview questions.

Description Solution Video 27:47

784. Letter Case Permutation

Medium75.7% acceptance7 companies

Given a string s, you can transform every letter individually to be lowercase or uppercase to create another string.

Return a list of all possible strings we could create. Return the output in any order.

Example 1:

Input: s = "a1b2"
Output: ["a1b2","a1B2","A1b2","A1B2"]

Example 2:

Input: s = "3z4"
Output: ["3z4","3Z4"]

Constraints:

1 <= s.length <= 12
s consists of lowercase English letters, uppercase English letters, and digits.

Asked at 7 companies

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27:47

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784. Letter Case Permutation

Asked at 7 companies

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