Check if One String Swap Can Make Strings Equal in Java

27 Mar 2025 | 2 min read

Two strings, str1 and str2, of the same length are given to us. Selecting two indices in a string, which need not be distinct, and switching the characters at these indices is known as a string swap. If, at most, one string swap can be done on precisely one of the strings to make both strings equal, then return true. Return false in the other case.

Example 1:

Input:

String str1 = "bank"

String str2 = "kanb"

Output: true

Explanation:

The easiest way to create a "bank" is to replace the first character with the last character of str2.

Example 2:

Input:

String str1 = "attack"

String str2 = "defend"

Output: false

Explanation:

A single-string switch will never be enough to equalize them.

Example 3:

Input:

String str1 = "kelb"

String str2 = "kelb"

Output: true

Explanation:

There is no need to perform a string swap because the two strings are already equal.

Naïve Approach

The approach determines if two strings can be equalized by exchanging a maximum of one pair of characters. It detects mismatched characters by looping across both strings. The characters are stored if there is precisely one mismatch; the function returns false if there are more than two mismatches or if the mismatched characters cannot be switched to make the strings equal. To ensure that the swap is valid, the procedure only returns true if there are precisely two or zero mismatches. The method provides an effective resolution with a linear time complexity of O(n), where n represents the string length.