01 Jan 2024•Python•Medium

letter combinations of a phone number

LeetCode 17: Letter Combinations of a Phone Number

i recently solved the letter combinations of a phone number problem on leetcode, and it's a great example of string, backtracking techniques. this medium problem tests your understanding of string, backtracking.

Problem Statement

[problem description would go here - you can add the actual problem statement]

My Approach

when i first saw this problem, i immediately thought about using backtracking. the key insight is [to be filled based on the specific problem].

Initial Thoughts

i started by thinking about different approaches:

**brute force** - [brute force approach description]
**optimized approach** - Backtracking
**alternative approach** - [alternative approach if applicable]

Solution Strategy

i decided to use a Backtracking with the following strategy:

**[step 1]** - [description]
**[step 2]** - [description]
**[step 3]** - [description]

My Solution

def solution(self, params):
# implementation would go here
pass

Code Breakdown

let me walk through how this solution works:

1. [Section 1]

# code snippet

[explanation]

2. [Section 2]

# code snippet

[explanation]

Example Walkthrough

let's trace through an example:

[step-by-step walkthrough]

Time and Space Complexity

**time complexity:** [complexity analysis]
**space complexity:** [complexity analysis]

Key Insights

**[insight 1]** - [explanation]
**[insight 2]** - [explanation]
**[insight 3]** - [explanation]

Alternative Approaches

i also considered:

**[alternative 1]** - [description]
**[alternative 2]** - [description]

Edge Cases to Consider

**[edge case 1]** - [description]
**[edge case 2]** - [description]
**[edge case 3]** - [description]

Lessons Learned

this problem taught me:

[lesson 1]
[lesson 2]
[lesson 3]

Conclusion

the letter combinations of a phone number problem is a great exercise in string, backtracking. the key insight is [key takeaway].

you can find my complete solution on [leetcode](https://leetcode.com/problems/letter-combinations-of-a-phone-number/solutions/1234569/letter-combinations-of-a-phone-number-solution-by-1cbyc).

---

*this is part of my leetcode problem-solving series. i'm documenting my solutions to help others learn and to track my own progress.*

Solution files

PHP•letter-combinations-of-a-phone-number/solution.php

<?php
class Solution {
    private $digitToLetters = [
        class="syntax-string">'class="syntax-number">2' => [class="syntax-string">'a', class="syntax-string">'b', class="syntax-string">'c'],
        class="syntax-string">'class="syntax-number">3' => [class="syntax-string">'d', class="syntax-string">'e', class="syntax-string">'f'],
        class="syntax-string">'class="syntax-number">4' => [class="syntax-string">'g', class="syntax-string">'h', class="syntax-string">'i'],
        class="syntax-string">'class="syntax-number">5' => [class="syntax-string">'j', class="syntax-string">'k', class="syntax-string">'l'],
        class="syntax-string">'class="syntax-number">6' => [class="syntax-string">'m', class="syntax-string">'n', class="syntax-string">'o'],
        class="syntax-string">'class="syntax-number">7' => [class="syntax-string">'p', class="syntax-string">'q', class="syntax-string">'r', class="syntax-string">'s'],
        class="syntax-string">'class="syntax-number">8' => [class="syntax-string">'t', class="syntax-string">'u', class="syntax-string">'v'],
        class="syntax-string">'class="syntax-number">9' => [class="syntax-string">'w', class="syntax-string">'x', class="syntax-string">'y', class="syntax-string">'z'],
    ];

    function letterCombinations($digits) {
        if (empty($digits)) {
            return [];
        }

        return $this->generateCombinations($digits);
    }

    private function generateCombinations($digits) {
        if (empty($digits)) {
            return [class="syntax-string">""];
        }

        $currentDigit = $digits[class="syntax-number">0];
        $remainingDigits = substr($digits, class="syntax-number">1);
        $currentLetters = $this->digitToLetters[$currentDigit];
        $combinationsForRemaining = $this->generateCombinations($remainingDigits);

        $result = [];
        foreach ($currentLetters as $letter) {
            foreach ($combinationsForRemaining as $combination) {
                $result[] = $letter . $combination;
            }
        }

        return $result;
    }
}

$solution = new Solution();
print_r($solution->letterCombinations(class="syntax-string">"class="syntax-number">23"));
print_r($solution->letterCombinations(class="syntax-string">""));
print_r($solution->letterCombinations(class="syntax-string">"class="syntax-number">2"));
?>