To solve this problem, check each word in the array and try to find its reverse in the list. The challenge lies in efficiently identifying such pairs. Using a hash table speeds up the search process, leading to an optimal solution.
Problem Statement
Given a list of distinct strings, each with two characters, your task is to find how many pairs of strings can be formed such that one string is the reverse of the other.
Return the maximum number of such pairs possible from the given array of strings.
Examples
Example 1
Input: words = ["cd","ac","dc","ca","zz"]
Output: 2
In this example, we can form 2 pair of strings in the following way:
- We pair the 0th string with the 2nd string, as the reversed string of word[0] is "dc" and is equal to words[2].
- We pair the 1st string with the 3rd string, as the reversed string of word[1] is "ca" and is equal to words[3]. It can be proven that 2 is the maximum number of pairs that can be formed.
Example 2
Input: words = ["ab","ba","cc"]
Output: 1
In this example, we can form 1 pair of strings in the following way:
- We pair the 0th string with the 1st string, as the reversed string of words[1] is "ab" and is equal to words[0]. It can be proven that 1 is the maximum number of pairs that can be formed.
Example 3
Input: words = ["aa","ab"]
Output: 0
In this example, we are unable to form any pair of strings.
Constraints
- 1 <= words.length <= 50
- words[i].length == 2
- words consists of distinct strings.
- words[i] contains only lowercase English letters.
Solution Approach
Hash Table Lookup
Use a hash table to store the words from the array, then for each word, check if its reverse exists in the hash table. Count each valid pair, and ensure you don’t count pairs more than once.
Array Scanning
Scan through the array and for each word, check its reverse using the hash table. This ensures an efficient solution as opposed to brute force.
Distinct Strings
Since all strings are distinct, checking for the reverse of each word ensures no duplicates are counted, simplifying the logic and improving efficiency.
Complexity Analysis
| Metric | Value |
|---|---|
| Time | Depends on the final approach |
| Space | Depends on the final approach |
The time complexity depends on the number of words and the lookups in the hash table. The space complexity depends on the size of the hash table used to store the words.
What Interviewers Usually Probe
- Look for candidates who use a hash table for efficient lookups.
- Evaluate if the candidate ensures no duplicate pairs are counted.
- Assess the candidate’s understanding of leveraging hash tables to optimize array scanning.
Common Pitfalls or Variants
Common pitfalls
- Forgetting to reverse each word and check for its pair.
- Not using a hash table, resulting in slow lookups.
- Miscounting pairs when both words are part of the same pair, leading to duplication.
Follow-up variants
- Extend the problem by increasing the length of the words.
- Use a different data structure to store the words, such as a set.
- Modify the problem by considering multi-character words.
How GhostInterview Helps
- GhostInterview’s solver can help you efficiently pair strings using hash tables, ensuring optimal performance.
- The solver guides you through array scanning techniques while preventing common pitfalls like duplicate counting.
- Through detailed problem breakdowns, GhostInterview helps reinforce your understanding of efficient lookups with hash tables.
Topic Pages
Related GhostInterview Pages
- LeetCode Interview Copilot - Use GhostInterview as a live solver when you want direct help with LeetCode-style coding questions.
- Coding Interview Assistant - See how GhostInterview supports array, string, linked list, graph, and tree interview workflows.
- How GhostInterview Works - Review the screenshot, reasoning, and answer flow before using the solver in a live interview.
FAQ
What is the optimal way to solve 'Find Maximum Number of String Pairs'?
The optimal solution uses a hash table to store the strings and checks for the reverse of each word efficiently.
How do I prevent counting duplicate pairs?
By ensuring each pair is counted only once, for example by marking words that have already been paired.
What is the time complexity of this problem?
The time complexity depends on the number of words and the hash table lookups, typically O(n).
Can this problem be solved without a hash table?
Yes, but using a hash table ensures more efficient lookups and reduces time complexity.
What other data structures could be used for this problem?
You could use a set or a list, but hash tables provide the most efficient solution for this problem.
Need direct help with Find Maximum Number of String Pairs instead of spending more time grinding it?
Download GhostInterview when you want a LeetCode solver, not another long practice loop. Capture Find Maximum Number of String Pairs from a screenshot, get the answer path and complexity, and use supported stealth workflows that stay outside captured layers.
Capture the prompt fast instead of rewriting the problem by hand.
Get the solution path, trade-offs, and complexity summary in one pass.
Stay outside captured layers on supported screen-share workflows.
Stay in the same pattern family
Simulate a series of add and jump instructions on arrays to compute the final score efficiently using array scanning and hash lookup.
Open problem page#2766 Relocate MarblesRelocate marbles to new positions in an array by simulating moves, and return sorted occupied positions.
Open problem page#2707 Extra Characters in a StringThe problem asks for the minimum number of extra characters left after optimally breaking a string into substrings found in a dictionary.
Open problem page