5 Best Ways to Check if One String Swap Can Make Two Strings Equal in Python

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πŸ’‘ Problem Formulation: You are given two strings, and the task is to determine whether making a single swap in one of the strings can make them equal. For instance, swapping ‘a’ and ‘b’ in “converse” to form “converes” checks if it can match with “converes”. A successful algorithm should signal that one swap can indeed make both strings equal.

Method 1: Brute Force Swap Check

This method involves iterating through each character in the string and attempting a swap with every other character, then checking if the resulting string matches the second string. This approach is straightforward but may not be the most efficient for long strings.

Here’s an example:

def can_make_equal_by_swap(s1, s2):
    if s1 == s2:
        return True
    for i in range(len(s1)):
        for j in range(i + 1, len(s1)):
            swapped = s1[:i] + s1[j] + s1[i+1:j] + s1[i] + s1[j+1:]
            if swapped == s2:
                return True
    return False

print(can_make_equal_by_swap("converse", "converes"))

Output: True

This function, can_make_equal_by_swap, checks each possible pair of characters to swap in s1, then compares the result to s2. If a match is found, it returns True, otherwise False after trying all combinations.

Method 2: Tracking Differences

The tracking differences method checks the positions where the two strings differ and ensures these can be resolved with a single swap. This is more efficient than brute force as it reduces the number of comparisons.

Here’s an example:

def can_make_equal_by_one_swap(s1, s2):
    diffs = [[x, y] for x, y in zip(s1, s2) if x != y]
    if len(diffs) == 2 and diffs[0][::-1] == diffs[1]:
        return True
    return len(diffs) == 0

print(can_make_equal_by_one_swap("converse", "converes"))

Output: True

The function can_make_equal_by_one_swap creates a list of characters that are different between s1 and s2. It then checks if the list size is two and the two differences can be swapped to make the strings equal.

Method 3: Optimized Comparison with Early Termination

By optimizing comparisons and introducing early termination when more than two differences are found, this method efficiently identifies if a single swap can make the strings equal. It’s more efficient than previous methods, as it stops as soon as it finds more than two mismatches.

Here’s an example:

def can_swap_to_equal(s1, s2):
    diffs = []
    for x, y in zip(s1, s2):
        if x != y:
            if len(diffs) >= 2:
                return False
            diffs.append((x, y))
    return len(diffs) == 2 and diffs[0] == diffs[1][::-1]

print(can_swap_to_equal("converse", "converes"))

Output: True

The function can_swap_to_equal uses early termination to avoid unnecessary comparisons after detecting more than two differences between s1 and s2. This method quickly identifies if the strings can be made equal with a single swap.

Method 4: Set-Based Validation

This method utilizes sets to identify the differences and assess if a single swap can resolve them. It’s suited for strings with unique characters and can quickly dismiss cases with too many differences.

Here’s an example:

def set_based_single_swap(s1, s2):
    if len(s1) != len(s2) or sorted(s1) != sorted(s2):
        return False
    return sum(a != b for a, b in zip(s1, s2)) == 2

print(set_based_single_swap("converse", "converes"))

Output: True

By first checking if the sorted versions of s1 and s2 are equal, the function set_based_single_swap ensures both strings contain the same characters. It then requires exactly two positions of difference to validate the swap condition.

Bonus One-Liner Method 5: Lambda Function with Conditional Expressions

For those who love compact code, a lambda function with a conditional expression can accomplish the task in a single line. This method trades readability for brevity and is not recommended for beginner programmers.

Here’s an example:

check_swap = lambda a, b: len(a) == len(b) and sum(x != y for x, y in zip(a, b)) in [0, 2]
print(check_swap("converse", "converes"))

Output: True

This one-liner lambda function, check_swap, restricts the possible number of differences between the strings a and b to exactly zero or two, with the additional constraint that both strings are of equal length.


  • Method 1: Brute Force Swap Check. Strengths: Conceptually simple, does not require any preconditions. Weaknesses: Not efficient for long strings, has O(n^2) complexity.
  • Method 2: Tracking Differences. Strengths: More efficient, only requires a single loop. Weakness: Still checks every character, resulting in O(n) complexity.
  • Method 3: Optimized Comparison with Early Termination. Strengths: Efficient and fast, early termination improves performance. Weakness: More complex logic than Method 2.
  • Method 4: Set-Based Validation. Strengths: Very fast for strings with unique characters, quickly dismisses non-matching strings. Weaknesses: Requires characters to be unique and sorting, which has O(n log n) complexity.
  • Method 5: Lambda Function with Conditional Expressions. Strengths: Compact and concise. Weaknesses: Less readable and not intuitive for those unfamiliar with Python idioms.