859.buddy-strings.md

859. Buddy Strings

There are some possible scenarios:

• The two strings have different lengths.
• s == goal: See below
• diff(s, goal) = 1: False, ab, aa.
• diff(s, goal) = 2: Check if the two different characters can be swapped.
• diff(s, goal) > 2: False

This problem has a very particular edge case: aa and aa. In this case, we can swap the two as and get the same string. The two a's are considered as the different characters. So we can swap them and get the same string.

Count the Difference

fun buddyStrings(s: String, goal: String): Boolean {
    if (s.length != goal.length) return false

    // Handle the special edge case, if there are (multiple) duplicate characters when s == goal
    // s = "aa"
    // goal = "aa"
    if (s == goal) {
        val count = IntArray(26)
        for (c in s) {
            count[c - 'a']++
            if (count[c - 'a'] > 1) return true
        }
        return false
    }

    // Otherwise, we need to find the two different characters
    var diff1: Pair<Char, Char>? = null
    var diff2: Pair<Char, Char>? = null
    for (i in 0 until s.length) {
        if (s[i] != goal[i]) {
            if (diff1 == null) diff1 = s[i] to goal[i]
            else if (diff2 == null) diff2 = s[i] to goal[i]
            else {
                // More than 2 differences
                return false
            }
        }
    }
    if (diff1 != null && diff2 != null) {
        // Check if we can swap the two different characters
        return diff1.first == diff2.second && diff1.second == diff2.first
    } else {
        // Less than 2 differences
        return false
    }
}