1790. Check if One String Swap Can Make Strings Equal

Method 1

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.

Example 1

Input: s1 = "bank", s2 = "kanb"
Output: true
Explanation: For example, swap the first character with the last character of s2 to make "bank".

Example 2

Input: s1 = "attack", s2 = "defend"
Output: false
Explanation: It is impossible to make them equal with one string swap.

Example 3

Input: s1 = "kelb", s2 = "kelb"
Output: true
Explanation: The two strings are already equal, so no string swap operation is required.

Method 1

【O(n) time | O(1) space】

package Leetcode.String;

/**
 * @author zhengxingxing
 * @date 2025/02/05
 */
public class CheckIfOneStringSwapCanMakeStringsEqual {
    public static boolean areAlmostEqual(String s1, String s2) {
        // If the lengths of the two strings are not equal, return false
        if (s1.length() != s2.length()) {
            return false;
        }

        // Variables to store indices of different characters
        // i1 will store the index of the first differing character
        // i2 will store the index of the second differing character
        int i1 = -1;
        int i2 = -1;

        // Iterate through each character of both strings
        for (int i = 0; i < s1.length(); i++) {
            // If characters at the current index are the same, continue to the next index
            if (s1.charAt(i) == s2.charAt(i)) {
                continue;
            }

            // If we already found two differences and another one is found,
            // then we need more than one swap, hence return false
            if (i2 != -1) {
                return false;
            }

            // If this is the first differing character, store its index
            if (i1 == -1) {
                i1 = i;
            } else {
                // This is the second differing character, store its index
                i2 = i;
            }
        }

        // If no differing characters were found, it means both strings are already equal
        if (i1 == -1 && i2 == -1) {
            return true;
        }

        // If we found only one differing character, return false
        // Because a single swap can only fix two differing characters
        if (i2 == -1) {
            return false;
        }

        // Finally, check if swapping the characters at these two indices
        // would make the two strings equal
        return s1.charAt(i1) == s2.charAt(i2) && s1.charAt(i2) == s2.charAt(i1);
    }

    public static void main(String[] args) {
        // Test case 1
        String s1 = "bank";
        String s2 = "kanb";
        System.out.println("Test Case 1: " + areAlmostEqual(s1, s2)); // Expected output: true

        // Test case 2
        s1 = "attack";
        s2 = "defend";
        System.out.println("Test Case 2: " + areAlmostEqual(s1, s2)); // Expected output: false

        // Test case 3
        s1 = "kelb";
        s2 = "kelb";
        System.out.println("Test Case 3: " + areAlmostEqual(s1, s2)); // Expected output: true

        // Test case 4 (where there's a possible swap)
        s1 = "abcd";
        s2 = "badc";
        System.out.println("Test Case 4: " + areAlmostEqual(s1, s2)); // Expected output: false

        // Test case 5 (where too many differences)
        s1 = "abc";
        s2 = "xyz";
        System.out.println("Test Case 5: " + areAlmostEqual(s1, s2)); // Expected output: false
    }
}

Method 1

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