What is the optimal time complexity of solving Letter Combinations of a Phone Number?

The optimized solution achieves a time complexity of O(n) by using sorting, mathematical shortcuts, or hash table lookups.

What is the auxiliary space complexity for Letter Combinations of a Phone Number?

The space complexity is O(n) since we only use a minimal/constant amount of extra memory for tracking variables (or auxiliary space if dynamic structures are allocated).

What are the typical edge cases we must handle in Letter Combinations of a Phone Number?

Key edge cases include empty collections, negative or large bounds causing integer overflow, arrays with only one element, or inputs where target criteria are not met.

How can we optimize the brute force approach for Letter Combinations of a Phone Number?

Brute force methods often inspect all possible combinations. We optimize this by sorting the input list to enable binary search, or tracking processed items in a set to avoid redundant nested loop lookups.

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Letter Combinations of a Phone Number

Learn how to solve the 'Letter Combinations of a Phone Number' problem. This detailed resource details brute force and optimized approaches.

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Problem Statement

Easy

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.

A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

2: abc, 3: def, 4: ghi, 5: jkl, 6: mno, 7: pqrs, 8: tuv, 9: wxyz

Implement a function letterCombinations(digits: str) -> list.

Constraints

•0 <= digits.length <= 4
•digits[i] is a digit in the range ['2', '9']

Examples

Example 1

Input

"23"

Output

["ad","ae","af","bd","be","bf","cd","ce","cf"]

Explanation

Digit 2 maps to 'abc' and digit 3 maps to 'def'. All combinations of one letter from each digit are generated.

Example 2

Input

""

Output

[]

Explanation

Empty input produces no combinations.

Example 3

Input

"2"

Output

["a","b","c"]

Explanation

Digit 2 maps to 'abc'.

Need a Hint?

Analyze the input constraints. Try sorting first (O(n log n)) or using a hash map/set to track seen elements in O(n) time.

Edge Cases to Watch

Empty list or null input variables
Single item lists/arrays
Extremely large input bounds causing integer or stack overflow

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