main.go

// Source: https://leetcode.com/problems/check-if-one-string-swap-can-make-strings-equal
// Title: Check if One String Swap Can Make Strings Equal
// Difficulty: Easy
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// 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.
// ```
//
// **Constraints:**
//
// - `1 <= s1.length, s2.length <= 100`
// - `s1.length == s2.length`
// - `s1` and `s2` consist of only lowercase English letters.
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

func areAlmostEqual(s1 string, s2 string) bool {
	n := len(s1)

	findNextDiffer := func(idx int) int {
		for ; idx < n; idx++ {
			if s1[idx] != s2[idx] {
				return idx
			}
		}
		return -1
	}

	firstIdx := findNextDiffer(0)
	if firstIdx == -1 { // s1 == s2
		return true
	}
	first1, first2 := s1[firstIdx], s2[firstIdx]

	secondIdx := findNextDiffer(firstIdx + 1)
	if secondIdx == -1 { // only one different character
		return false
	}
	second1, second2 := s1[secondIdx], s2[secondIdx]

	if first1 != second2 || first2 != second1 { // can't swap
		return false
	}

	thirdIdx := findNextDiffer(secondIdx + 1)
	return thirdIdx == -1 // rest characters should be equal
}