859. 亲密字符串

public class Solution {
  public bool BuddyStrings(string A, string B)
        {
            bool flag = false;
            int[] n =new int[26];
            if (A.Length == B.Length)
            {
                int count = 0;
                //判断是否可以通过交换两个字母得到相等结果
                int index = -1;
                for (int i = 0; i < A.Length; i++)
                {
                    int bet = A[i] - 'a';
                    n[bet]++;
                    if (n[bet] >= 2)
                    {
                        flag = true;
                    }
                    if (A[i] != B[i])
                    {
                        if (index == -1)
                        {
                            index = i;
                        }
                        if (A[index] == B[i] && A[i] == B[index])
                        {
                            flag = true;
                        }
                        else
                        {
                            flag = false;
                        }
                        count++;
                    }
                }
                if (flag && count <= 2)
                {
                    return true;
                }
            }
            return false;
        }
}