859. Buddy Strings

Given two strings A and B of lowercase letters, return true if and only if we can swap two letters in A so that the result equals B.

Example 1:

Input: A = “ab”, B = “ba”
Output: true

Example 2:

Input: A = “ab”, B = “ab”
Output: false

Example 3:

Input: A = “aa”, B = “aa”
Output: true

Example 4:

Input: A = “aaaaaaabc”, B = “aaaaaaacb”
Output: true

Example 5:

Input: A = “”, B = “aa”
Output: false
 

Note:

0 <= A.length <= 20000
0 <= B.length <= 20000
A and B consist only of lowercase letters.

Code:

If A.length() != B.length(): no possible swap

If A == B, we need swap two same characters. Check is duplicated char in A.

In other cases, we find index for A[i] != B[i]. There should be only 2 diffs and it’s our one swap.

public class Solution {
    public bool BuddyStrings(string A, string B) {
        if (A.Length != B.Length) return false;
        if (A.Equals(B)) {
            HashSet<char> s = new HashSet<char>();
            foreach(var c in A) s.Add(c);
            return s.Count < A.Length;
        }
        List<int> dif = new List<int>();
        for (int i = 0; i < A.Length; ++i) 
            if (A[i] != B[i]) 
                dif.Add(i);
        return dif.Count == 2 && A[dif[0]] == B[dif[1]] && A[dif[1]] == B[dif[0]];
    }
}