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Palindrome Pairs

Certification: Advanced Level Accuracy: 0% Submissions: 0 Points: 15

Write a C program to find all pairs of unique indices (i, j) such that the concatenation of words[i] + words[j] forms a palindrome. Given a list of unique words, you need to return all the pairs of indices where concatenating the words at those indices results in a palindrome string.

Example 1
    
      
  • Input: words = ["lls", "s", "sssll"]
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  • Output: [[0,1], [1,0], [2,1]]
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  • Explanation: 
     Step 1: Check all possible pairs for palindrome concatenation

     Step 2: words[0] + words[1] = "lls" + "s" = "llss" (not palindrome)

     Step 3: words[1] + words[0] = "s" + "lls" = "slls" (not palindrome)

     Step 4: words[0] + words[1] = "lls" + "s" = "llss", words[1] + words[0] = "s" + "lls" = "slls", words[2] + words[1] = "sssll" + "s" = "ssslls" (palindrome)
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Example 2
    
      
  • Input: words = ["abcd", "dcba", "lls", "s", "sssll"]
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  • Output: [[0,1], [1,0], [3,2], [2,4]]
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  • Explanation: 
     Step 1: "abcd" + "dcba" = "abcddcba" (palindrome)

     Step 2: "dcba" + "abcd" = "dcbaabcd" (palindrome)

     Step 3: "s" + "lls" = "slls" (not palindrome), but reverse combinations work

     Step 4: Multiple valid palindrome pairs found
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Constraints
    
      
  • 1 <= words.length <= 5000
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  • 0 <= words[i].length <= 300
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  • words[i] consists of lowercase English letters
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  • All words are unique
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  • Time Complexity: O(n^2 * m) where n is number of words, m is average length
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  • Space Complexity: O(1) excluding output array
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Solution

Solution Hints

    
      
  • Check every pair of words (i, j) where i != j
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  • For each pair, concatenate words[i] + words[j] and check if it's a palindrome
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  • Use a helper function to verify if a string is a palindrome
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  • Store valid pairs as [i, j] indices in the result array
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  • Iterate through all combinations systematically to avoid missing pairs
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  • Handle edge cases like empty strings appropriately
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Steps to solve by this approach:

 Step 1: Create a helper function to check if a given string is a palindrome using two pointers
 Step 2: Initialize result array and counter to store palindrome pairs
 Step 3: Use nested loops to check all possible pairs (i, j) where i != j
 Step 4: For each pair, concatenate words[i] + words[j] to form a new string
 Step 5: Use the palindrome helper function to check if concatenated string is palindrome
 Step 6: If palindrome is found, store the indices [i, j] in result array
 Step 7: Return all found palindrome pairs with their respective indices

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