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Check if One String Swap Can Make Strings Equal

Difficulty: Easy

Problem Description

You are given two strings s1 and s2 of equal length. A string swap is an operation where you choose two indices in a string (not necessarily different) and swap the characters at these indices. Return true if it is possible to make both strings equal by performing at most one string swap on exactly one of the strings. Otherwise, return false.

Key Insights

  • The two strings must be of equal length.
  • If the strings are already equal, no swaps are needed, and the answer is true.
  • If the strings differ in more than two characters, it's impossible to make them equal with a single swap.
  • If the strings differ in exactly two characters, we can check if swapping those characters in one string can make it equal to the other.

Space and Time Complexity

Time Complexity: O(n)
Space Complexity: O(1)

Solution

The solution involves comparing the two strings character by character. We can maintain a list of indices where the characters differ. If the number of differing positions is zero, we return true (the strings are equal). If there are exactly two differing positions, we check if swapping the characters at those positions in one string makes it equal to the other. If there are more than two differing positions, we return false.

Code Solutions

def areAlmostEqual(s1: str, s2: str) -> bool:
    if s1 == s2:
        return True
    
    # Store the indices where the characters differ
    diff = []
    for i in range(len(s1)):
        if s1[i] != s2[i]:
            diff.append(i)
    
    # If there are exactly 2 differences, check if swapping makes them equal
    if len(diff) == 2:
        return s1[diff[0]] == s2[diff[1]] and s1[diff[1]] == s2[diff[0]]
    
    return False
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