Buddy Strings

Given two strings s and goal, return true if you can swap two letters in s so the result is equal to goal, 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 s[i] and s[j].

• For example, swapping at indices 0 and 2 in "abcd" results in "cbad".

Example 1:

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

Example 2:

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

Example 3:

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

Constraints:

• 1 <= s.length, goal.length <= 2 * 104
• s and goal consist of lowercase letters.

思路：

• Using 2 indices diff1, diff2 to store up to 2 different places between string A and string B.
• If there are more than 2 different places -> Invalid.
• If there are 2 different places -> Compare A[diff1] vs B[diff2] and A[diff2] vs B[diff1].
• If there is only 1 different places -> Invalid.
• If no difference between A and B then check if A contains at least 1 duplicate letters so that we can swap them.
  • Example 1: A = "ab", B = "ab"
  • Example 2: A = "aab", B = "aab",

class Solution {
    public boolean buddyStrings(String s, String goal) {
        if(s.length() != goal.length()) {
            return false;
        }
        int first = -1; int second = -1;
        HashSet<Character> set = new HashSet<>();
        for(int i = 0; i < s.length(); i++) {
            if(s.charAt(i) != goal.charAt(i)) {
                if(first == -1) {
                    first = i;
                } else if(second == -1) {
                    second = i;
                } else { // 3 diff;
                    return false;
                }
            }
            set.add(s.charAt(i));
        }
        
        if(first != -1 && second != -1) {
            return s.charAt(first) == goal.charAt(second) 
                && s.charAt(second) == goal.charAt(first);
        }
        if(first != -1 && second == -1) {
            return false;
        }
         
        return set.size() < s.length(); // set size 小于len，说明有至少一个重复，可以互换；
    }
}