Python program to count the pairs of reverse strings

When it is required to count the pairs of reverse strings, we need to check if each string in a list has its reverse counterpart present. A simple iteration is used to compare strings with their reversed versions.

Example

Below is a demonstration of counting reverse string pairs ?

def count_reverse_pairs(string_list):
    count = 0
    visited = set()
    
    for i in range(len(string_list)):
        if i in visited:
            continue
            
        current_string = string_list[i]
        reverse_string = current_string[::-1]
        
        # Check if reverse exists in remaining elements
        for j in range(i + 1, len(string_list)):
            if j in visited:
                continue
                
            if string_list[j] == reverse_string:
                count += 1
                visited.add(i)
                visited.add(j)
                print(f"Pair found: '{current_string}' and '{reverse_string}'")
                break
    
    return count

# Example usage
my_list = ["abc", "cba", "hello", "olleh", "python", "world"]
print("The list is:")
print(my_list)

pairs_count = count_reverse_pairs(my_list)
print(f"\nTotal reverse string pairs: {pairs_count}")

The list is:
['abc', 'cba', 'hello', 'olleh', 'python', 'world']
Pair found: 'abc' and 'cba'
Pair found: 'hello' and 'olleh'

Total reverse string pairs: 2

Method Using Dictionary for Efficiency

For better performance with larger lists, we can use a dictionary to store string frequencies ?

def count_reverse_pairs_optimized(string_list):
    string_count = {}
    pairs = 0
    
    # Count frequency of each string
    for string in string_list:
        string_count[string] = string_count.get(string, 0) + 1
    
    processed = set()
    
    for string in string_list:
        if string in processed:
            continue
            
        reverse_string = string[::-1]
        
        if string == reverse_string:
            # Palindrome case
            pairs += string_count[string] // 2
        elif reverse_string in string_count and reverse_string not in processed:
            # Regular reverse pair
            pairs += min(string_count[string], string_count[reverse_string])
        
        processed.add(string)
        processed.add(reverse_string)
    
    return pairs

# Example with duplicates
my_list = ["abc", "cba", "abc", "cba", "dad", "dad"]
print("The list is:")
print(my_list)

pairs_count = count_reverse_pairs_optimized(my_list)
print(f"Total reverse string pairs: {pairs_count}")

The list is:
['abc', 'cba', 'abc', 'cba', 'dad', 'dad']
Total reverse string pairs: 3

Comparison

Conclusion

Use the simple iteration method for small lists where you want to see individual pairs. Use the dictionary method for better performance with larger datasets or when dealing with duplicate strings.