LeetCode/cs/check-if-one-string-swap-can-make-strings-equal.cs

using Pairing = (char, char);

public class Solution {
    private static readonly Pairing UNSET = ('\0', '\0');

    private record Diff(int Count, Pairing First, Pairing Second) {
        public Diff() : this(0, UNSET, UNSET) { }

        public Diff Update(char l, char r) {
            if (l == r) {
                return this;
            }

            return new Diff(
                Count + 1,
                First == UNSET ? (l, r) : First,
                First != UNSET && Second == UNSET ? (l, r) : Second
            );
        }

        public bool CanSwap => Count == 0 || (
            Count == 2
            && First.Item1 == Second.Item2
            && First.Item2 == Second.Item1
        );
    }

    public bool AreAlmostEqual(string s1, string s2) =>
        s1.Zip(s2).Aggregate(new Diff(), (acc, chars) => {
            var (l, r) = chars;
            return acc.Update(l, r);
        }).CanSwap;
}