859. Buddy Strings

Given two strings A and B of lowercase letters, return true if you can swap two letters in A so the result is equal to B, otherwise, return false.

Swapping letters is defined as taking two indices i and j (0-indexed) such that i != j and swapping the characters at A[i] and A[j]. For example, swapping at indices 0 and 2 in "abcd" results in "cbad".

Example 1:

Input: A = "ab", B = "ba"
Output: true
Explanation: You can swap A[0] = 'a' and A[1] = 'b' to get "ba", which is equal to B.

Example 2:

Input: A = "ab", B = "ab"
Output: false
Explanation: The only letters you can swap are A[0] = 'a' and A[1] = 'b', which results in "ba" != B.

Example 3:

Input: A = "aa", B = "aa"
Output: true
Explanation: You can swap A[0] = 'a' and A[1] = 'a' to get "aa", which is equal to B.

Example 4:

Input: A = "aaaaaaabc", B = "aaaaaaacb"
Output: true

Example 5:

Input: A = "", B = "aa"
Output: false

Constraints:

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

class Solution {
    public boolean buddyStrings(String A, String B) {
        int n1 = A.length(), n2 = B.length();
        if(n1 != n2 || n1 == 0 || n2 == 0) return false;
        if(A.equals(B)) {
            int[] count = new int[26];
            for(char c : A.toCharArray()) count[c - 'a']++;
            
            for(int i : count) {
                if(i > 1) return true;
            }
            return false;
        }
        else {
            int fir = -1, sec = -1;
            for(int i = 0; i < n1; i++) {
                if(A.charAt(i) != B.charAt(i)) {
                    if(fir == -1) fir = i;
                    else if(sec == -1) sec = i;
                    else return false;
                }
            }
            return sec != -1 && A.charAt(fir) == B.charAt(sec) && A.charAt(sec) == B.charAt(fir);
        }        
    }
}

这也能是easy？泥头车创撕你。

corner case很多，首先当两个相等，判断下里面有没有一个char出现了两次或以上，如果有就返回true，否则返回false，因为不可能符合题意。

如果不相等，也要看是不是只出现了两个位置不相等，而且交换后相等，所以只能判断到第二个不相等的，如果不相等的小于2或者大于2，都返回错误。只有两个不相等的时候，且交换他俩后相等，才能返回true。