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Letter Combinations of a Phone Number

Certification: Intermediate Level Accuracy: 0% Submissions: 0 Points: 10

Write a C program to find all possible letter combinations that the numbers of a given string could represent on a telephone keypad. Given a string containing digits from 2-9, return all possible letter combinations that the number could represent. The mapping of digits to letters is the same as on telephone buttons (as shown below): - 2: abc - 3: def - 4: ghi - 5: jkl - 6: mno - 7: pqrs - 8: tuv - 9: wxyz

Example 1
    
  
  • Input: digits = "23"
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  • Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]
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  • Explanation: 
    The digit '2' corresponds to letters 'a', 'b', 'c'. The digit '3' corresponds to letters 'd', 'e', 'f'. 
    All possible combinations are: "ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf".
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Example 2
    
  
  • Input: digits = "5"
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  • Output: ["j", "k", "l"]
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  • Explanation: 
    The digit '5' corresponds to letters 'j', 'k', 'l'. 
    Since there is only one digit, the combinations are simply the letters themselves.
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Constraints
    
  
  • 0 <= digits.length <= 4
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  • digits[i] is a digit in the range ['2', '9']
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  • Time Complexity: O(4^n) where n is the length of digits
    • 
  
  • Space Complexity: O(n) for recursion stack
    •
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Hint
Solution

Solution Hints

    
  
  • Use a mapping of digits to their corresponding letters
    • 
  
  • Use backtracking or recursion to generate all possible combinations
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  • For each digit, try all its corresponding letters and proceed to the next digit
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  • Use a temporary string to build each combination
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  • When the combination's length equals the input's length, add it to the result
    •

Steps to solve by this approach:

 Step 1: Create a mapping from digits to their corresponding letters on a phone keypad.

 Step 2: Use a recursive backtracking approach to generate all possible combinations.

 Step 3: Calculate the maximum possible number of combinations to allocate memory for the result.

 Step 4: For each digit, try each of its corresponding letters.

 Step 5: Build the combination string character by character during recursion.

 Step 6: When we reach the end of the digits string, add the current combination to our result.

 Step 7: Handle edge cases like empty input strings, and properly manage memory allocation and deallocation.

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