# 859. Buddy Strings

## Description

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.

## Solutions

• class Solution {
public boolean buddyStrings(String s, String goal) {
int m = s.length(), n = goal.length();
if (m != n) {
return false;
}
int diff = 0;
int[] cnt1 = new int[26];
int[] cnt2 = new int[26];
for (int i = 0; i < n; ++i) {
int a = s.charAt(i), b = goal.charAt(i);
++cnt1[a - 'a'];
++cnt2[b - 'a'];
if (a != b) {
++diff;
}
}
boolean f = false;
for (int i = 0; i < 26; ++i) {
if (cnt1[i] != cnt2[i]) {
return false;
}
if (cnt1[i] > 1) {
f = true;
}
}
return diff == 2 || (diff == 0 && f);
}
}

• class Solution {
public:
bool buddyStrings(string s, string goal) {
int m = s.size(), n = goal.size();
if (m != n) return false;
int diff = 0;
vector<int> cnt1(26);
vector<int> cnt2(26);
for (int i = 0; i < n; ++i) {
++cnt1[s[i] - 'a'];
++cnt2[goal[i] - 'a'];
if (s[i] != goal[i]) ++diff;
}
bool f = false;
for (int i = 0; i < 26; ++i) {
if (cnt1[i] != cnt2[i]) return false;
if (cnt1[i] > 1) f = true;
}
return diff == 2 || (diff == 0 && f);
}
};

• class Solution:
def buddyStrings(self, s: str, goal: str) -> bool:
m, n = len(s), len(goal)
if m != n:
return False
cnt1, cnt2 = Counter(s), Counter(goal)
if cnt1 != cnt2:
return False
diff = sum(s[i] != goal[i] for i in range(n))
return diff == 2 or (diff == 0 and any(v > 1 for v in cnt1.values()))


• func buddyStrings(s string, goal string) bool {
m, n := len(s), len(goal)
if m != n {
return false
}
diff := 0
cnt1 := make([]int, 26)
cnt2 := make([]int, 26)
for i := 0; i < n; i++ {
cnt1[s[i]-'a']++
cnt2[goal[i]-'a']++
if s[i] != goal[i] {
diff++
}
}
f := false
for i := 0; i < 26; i++ {
if cnt1[i] != cnt2[i] {
return false
}
if cnt1[i] > 1 {
f = true
}
}
return diff == 2 || (diff == 0 && f)
}

• function buddyStrings(s: string, goal: string): boolean {
const m = s.length;
const n = goal.length;
if (m != n) {
return false;
}
const cnt1 = new Array(26).fill(0);
const cnt2 = new Array(26).fill(0);
let diff = 0;
for (let i = 0; i < n; ++i) {
cnt1[s.charCodeAt(i) - 'a'.charCodeAt(0)]++;
cnt2[goal.charCodeAt(i) - 'a'.charCodeAt(0)]++;
if (s[i] != goal[i]) {
++diff;
}
}
for (let i = 0; i < 26; ++i) {
if (cnt1[i] != cnt2[i]) {
return false;
}
}
return diff == 2 || (diff == 0 && cnt1.some(v => v > 1));
}