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Palindrome Pairs

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

Write a C++ program to find all pairs of distinct indices i and j in an array of words such that the concatenation of words[i] + words[j] forms a palindrome.

Example 1
    
  
  • Input: words = ["bat", "tab", "cat"]
    • 
  
  • Output: [[0,1], [1,0]]
    • 
  
  • Explanation: 
      
      
    • The palindrome pairs are:
      • 
      
    • - "bat" + "tab" = "battab", which is a palindrome
      • 
      
    • - "tab" + "bat" = "tabbat", which is a palindrome
      • 
    
    •
Example 2
    
  
  • Input: words = ["abcd", "dcba", "lls", "s", "sssll"]
    • 
  
  • Output: [[0,1], [1,0], [2,4], [3,2]]
    • 
  
  • Explanation: 
      
      
    • The palindrome pairs are:
      • 
      
    • - "abcd" + "dcba" = "abcddcba", which is a palindrome
      • 
      
    • - "dcba" + "abcd" = "dcbaabcd", which is a palindrome
      • 
      
    • - "lls" + "sssll" = "llssssll", which is a palindrome
      • 
      
    • - "s" + "lls" = "slls", which is a palindrome
      • 
    
    •
Constraints
    
  
  • 1 ≤ words.length ≤ 5000
    • 
  
  • 0 ≤ words[i].length ≤ 300
    • 
  
  • words[i] consists of lowercase English letters
    • 
  
  • Time Complexity: O(n * k²) where n is the number of words and k is the maximum length of a word
    • 
  
  • Space Complexity: O(n * k)
    •
Hash MapIBMPwC
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Hint
Solution

Solution Hints

    
  
  • Use a hash map to store words and their indices for quick lookup
    • 
  
  • For each word, check if its reverse exists in the array
    • 
  
  • Also check if any prefix or suffix of the word forms a palindrome
    • 
  
  • Be careful to exclude the word from pairing with itself
    • 
  
  • Pay attention to empty strings, which can form palindromes with any palindromic word
    •

Steps to solve by this approach:

 Step 1: Create a hash map to store each word and its index for quick lookup.
 Step 2: For each word, check if its reverse exists in the array to form a palindrome.
 Step 3: Handle the special case of an empty string, which can form a palindrome with any palindromic word.
 Step 4: For each word, check all its prefixes. If a prefix is a palindrome, check if the reversed suffix exists in the array.
 Step 5: Similarly, check all suffixes. If a suffix is a palindrome, check if the reversed prefix exists in the array.
 Step 6: Avoid adding duplicates and pairing a word with itself.
 Step 7: Return all valid palindrome pairs found.

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